It is common for verbs to be implied in the Greek but not actually appear in the text. Often the context will make little sense without it or will make it clear which verb is to be used, as is the case here. This is standard in many languages even today (especially with the verb "to be"), but not in English.
Let me begin with an example in English:
I will go north, Mike south, and Ted east.
The future form of the verb "to go" is implied in the second and third phrases. It could easily be written:
I will go north, Mike will go south, and Ted will go east.
We may not write it the latter way because it is redundant, however. The 'redundancy threshold' is much lower in Greek than it is in English. If the context makes it clear which verb is to be used, it is often left out of subsequent phrases. V. 4 provides the clue as to which verb should be used in the context of v. 6. Each subsequent phrase gives its own context for the verb to be used.
The immediate preceding context (vv. 4-5, NET) states:
For just as in one body we have many members, and not all the members
serve the same function, so we who are many are one body in Christ,
and individually we are members who belong to one another.
The word translated as "function" is πρᾶξιν, the accusative singular form of πρᾶξις (praxis). According to the BDAG lexicon, this word means "a function implying sustained activity, acting, activity, function; ... performance of some deed, act, action, deed." It is this word which forms the context for using some form of activity ("doing") as the verb of choice in the subsequent context (vv. 6-8). The point is that we don't all serve the same purpose ("do the same activity") in the body, yet we are still one body.
So when we come to v. 6 and a list of "functions" follows, we know that the context is "doing" these "functions" within the body. Here is the text of v. 6 (from NA27):
ἔχοντες δὲ χαρίσματα κατὰ τὴν χάριν τὴν δοθεῖσαν ἡμῖν διάφορα, εἴτε
προφητείαν κατὰ τὴν ἀναλογίαν τῆς πίστεως....
Allow me to propose a literal translation:
And having grace-gifts that differ according to the grace given to us,
if prophecy according to the proportion of faith....
This sentence really doesn't make much sense without helping verbs. Granted, it may appear to make sense by piecing together non-italicized fragments from English translations, but the Greek is missing a verb in each clause, indicating that one is implied. "To be" doesn't make sense here, so we should look at the context. As I've already explained, the context is doing some form of activity in the one body.
The first clause in v. 6 is generally translated in two different ways. Some add an entire phrase to the first clause of v. 6 ("let us use them"), and others treat the participle as a verb such as the NET translation. It is standard to use temporal clauses such as "since we have," "while we have," or (less common) "having then" to translate the progressive participle along with the basic conjunction δὲ, but those who allow the participle to remain in effect must supply a helping phrase to make sense of the sentence, hence "let us use them." This is consistent with the context:
Since we have grace-gifts that differ according to the grace given to
us, we should do something with them.
As for the second phrase and consequent phrases through the end of v. 8, this verb is simply implied each time:
...if the gift is prophecy, use it according to the proportion of
And so on:
*If the gift is service, do something in the body by serving; if teaching, by
teaching; if exhortation, by exhorting; if
giving, by giving with sincerity; if leading, by leading
diligently; if showing mercy, by doing so cheerfully.*
Note the italics: verbs are implied everywhere. The verb from the immediate context can be used (as I did in most cases above), or the general verb "to do/use" can be supplied. In all of these phrases, the basic pattern is as follows:
If the gift is _____, do something with/use it according to/by
Sentences without verbs usually imply them, as is the case all throughout this passage.
Option 1 or 2?
That leaves us to now to address whether or not option 2 is a good translation. It should be kept in mind that most translators follow option 1, and we should heed this. The majority of translators do not believe anything is missing in the argument that requires annexing v. 6 into the sentence to complete it. In fact, the meaning may actually change and/or become unclear if this is done.
You mentioned that "after reading v. 4, in v. 5 we are expecting a second half to the sentence that explains that not all the members have the same function, but we don't get that." I don't think that is the case. The idea is that just as a physical body has many limbs and organs that serve different functions, so also we who are many (with our many different functions) are also one body and we all belong to one another. I believe the functions are implied by indicating that we are many (πολλοὶ), and most translators seem to agree.
Option 2 will take one of three routes:
- If the progressive participle in v. 6 is translated literally (completely leaving out the conjunction since it would not make sense in English), then v. 6 would further modify "us" but the sentence would not resolve correctly, it would be a fragment: ("For just as in one body we have many members, and not all the members serve the same function, so we who are many are one body in Christ, and individually we are members who belong to one another, having grace-gifts that differ according to the grace given to us; if prophecy..."). Technically this could be resolved (as I did by placing a semicolon after 'us'), but this leaves out the conjunction and thus ignores a 'break' in the flow of the sentence which is implied by the Greek.
- The other route is to translate the progressive participle and conjunction by adding in a temporal clause in English to make sense of it ("while/since we have"). This is the route you took in your question. The problem is that without adding a sentence break, this introduces an argument which changes the meaning of the paragraph. Option 1 is saying that we are members of one body and belong to one another, serving different functions. It goes on to say that if our gift is X, we should serve/do that function appropriately within the body. This route is actually saying that vv. 4-5 are only true because we have different gifts/functions. In other words, because we have different gifts (or only while we have them), we are one body who belongs to one another just like a physical body with its many functions. This is too much of an interpretive stretch and is also easily misunderstood.
- The third route is to make a distinction between "us" exercising different grace-gifts in the one body vs. differing functions among members of a physical body. The problem with this approach is that I believe it creates a superfluous dichotomy rather than showing a comparison of similarities. This translation is shown below:
For by the grace given to me I say to every one of you not to think
more highly of yourself than you ought to think, but to think with
sober discernment, to each as God has distributed a measure of faith.
For just as in one body we have many members, but not all the members
serve the same function, so we the many are one body in Christ, and
each one members of one another, but having different grace-gifts
according to the grace given to us, whether prophecy, according to the
proportion of faith....
While this could also be read in the way I believe you were insinuating by option 2, the ambiguity creates a lot more room for confusion. This translation flows well in English, but it seems to imply an unnecessary dichotomy and thus could be understood as an illogical argument (a false or redundant comparison).
With these objections in mind, I believe option 1 is still the best translation choice. I've shown how three "routes" are plausible for option 2 (with route three being the most plausible), but neither seem as strong as the reasoning for option 1 to me. Again, it should be borne in mind that modern scholarly translations all follow option 1, and the phrases are missing verbs which begs them to be filled by the context.