Short Answer: The MT's base text is to be preferred. The passage is best understood as a lament by God about the oppression of His tribe. His tribe is like a child oppressed by women. (The statement about "women ruling" should not be taken as a literal description of the historical situation.)
MT Text and Analysis
The MT records the following:
עַמִּי נֹגְשָׂיו מְעוֹלֵל וְנָשִׂים מָשְׁלוּ בוֹ
suggesting the following original text:
Note the following:
מְעוֹלֵל is a ms participle from עלל, which can mean to vex, to glean, to act the child, or to thrust.
Both עם and מְעוֹלֵל are masculine singular. While נֹגְשָׂיו is masculine plural, its suffix is also masculine singular. Thus, it literally reads something like: "My tribe (ms)... those (mp) pressing him (ms)... the one (ms) who acts the child. . ." or something similar. In other words, God seems to be lamenting over His people who are playing the part of an oppressed child.
As I just illustrated, the MT does not require an emendation in order to make it coherent (despite the popular opinion of translators.)
LXX Text and Analysis
The LXX records the following:
Λαος μου, οἱ πρακτορες ὑμων καλαμωνται ὑμας, και οἱ απαιτουντες κυριευουσιν ὑμων·
While this is difficult to back-translate precisely due to the use of some unusual vocabulary, we can safely note the following:
The back-translation of και οἱ απαιτουντες (LXX) would have looked identical in the original text to the text behind וְנָשִׂים (MT): ונשׂים. So this is not a text-critical issue; it is simply a matter of interpretation. In fact the only text-critical issues are in the phrase "οἱ πρακτορες ὑμων καλαμωνται ὑμας".
Even though λαος is singular, indicating the LXX translators understood God to be addressing His people as a single entity, ὑμων and ὑμας are plural. (This may lend credence to the notion that the LXX translators were willing to shift singulars to plurals in order to better convey their understanding of the text to their audience.)
Ὁἱ πρακτορες is plural, as is καλαμωνται, indicating the translators understood the passage to be about a plural subject carrying out an action against God's people. The translators identify the plural subjects as "your officers" (or something similar.) This does not necessarily suggest a different Hebrew text though; it is not difficult to see how the LXX translators might have interpreted "those pressing [God's nation]" as "your officers" -- especially in light of the context.
While the exact meaning of καλαμωνται is uncertain, it may, in light of Sirach, have the idea of "gathering", or it may, in light of a similar noun meaning "reed", have the idea of "beating with a reed". I'm going out on a limb with these speculations, but my point is that either way it's not hard to see how they may have gotten a verbal idea (either to the effect of "taking" or "beating") from a Hebrew participle which can mean "glean" or "thrust" (among other things).
So, while we do have clear indication that the LXX translators understood the text in a different way than some others have, there does not seem to be sufficient evidence that they had a different version of the Hebrew text before them as they translated.
What about the NET?
The NET makes some changes to the MT's base text, removing the following:
...and adding the following:
From there they divide up the letters differently:
עמי נגשׂימ עוללו ונשׂים משׁלו בו
...and then interpret the vowel points differently:
עַמִּי נֹגְשִׂים עֹלְלוּ וְנֹשִׁים מָשְׁלוּ בוֹ
(With the changes highlighted in bold.) In other words, the NET translators found the MT Hebrew hard to understand, so they changed it to make more sense (to them.) This is a sketchy procedure, and can quickly become an exercise in subjectivity if we're not careful. Yes, it's possible for so many errors to have been made in transmission, but likely? Let's just say, before we jump to such conclusions, we should make absolutely certain the text does not make sense as it stands, and as I indicated above, we cannot make such an assertion in this case. The MT can work just fine as it stands.
From the above analysis, it would seem the MT's base text is a safe text to work from. That still leaves us with the question of interpretation, though.
Should ונשׂים be rendered וְנָשִׂים or וְנֹשִׁים?
What kind of participle is מְעוֹלֵל and how should we interpret it?
Is there a relationship between the first statement and the second? If so, what is it?
As noted above, the first clause literally reads something like "My tribe (ms)... those (mp) pressing him (ms)... the one (ms) who acts the child. . ." Thus, it is God's people who are מְעוֹלֵל and not the oppressors. Now, what exactly were God's people (actively) doing? Were they vexing? Were they gleaning? Were they acting the child? Or were they thrusting? "Acting the child" seems to make the most sense here.
So we now have a translation something like this:
O My tribe (ms)...
those oppressing him who plays the child...
And נשׂים rule in him...
The two options under consideration for נשׂים are נָשִׂים (women) and נֹשִׁים (creditors). At this point, creditors wouldn't make much sense; there is nothing in the context to suggest debt was in view. In contrast, women makes a lot of sense here. God's tribe is described as "acting the child" in relation to his oppressors. Placing "women" in parallel with "oppressors" not only continues the parental theme, but shocks the reader by describing these "oppressors" who rule over them as "women".
Our next task is to decide whether there were actually women ruling over them, or whether this is a word-picture designed to have some effect on the readers. The latter is to be preferred, not only because of the nature of their patriarchal society, but also clues from the immediate context. Isaiah 3:1-3 suggests that at the time of the prophecy men were leading, as usual, and Isaiah 3:16-26 suggests the women were playing the part of attractive objects as usual.