As the OP has already noted, the text of Rev 6 is disputed in many places in the first 8 verses. For simplicity, I will assume the text of the UBS5 is correct. [For details of the disputed passages and which MSS support each of the various readings, see UBS5.]
It is true that the command "Come" [ Ἔρχου ] could be directed to either the horse and its horseman, or, John as observer. Both are equally grammatically possible. [I also note that if the Byzantine text of "Come and See" is correct, then it makes the recipient of the command almost certainly, John.] However, we note the following:
- all the conversation in the immediately preceding text (Rev 5) is between the heavenly participants and does not include John.
- John is only spoken to by either his attending angel (Rev 17:7, 15, 19:9, 22:6), Jesus (Rev 1:10), or one of the 24 elders (Rev 7:13-17). That is, John is never spoken to by God or one of the four living creatures, unless Rev 6 is an exception.
Presumably, because of the influence of the Byzantine text of "Come and See", most commentators believe that the command is directed to John as observer (eg Albert Barnes).
Further, if John is instructed to "Come" he is never recorded as obeying the command. By contrast, each time on of the living creatures orders "Come", a horse and horseman obeys and comes out.
Note the comments of the Jamieson-Fausset-Brown Bible Commentary:
Come and see—One oldest manuscript, B, has "And see." But A, C, and Vulgate reject it. Alford rightly objects to English Version
reading: "Whither was John to come? Separated as he was by the glassy
sea from the throne, was he to cross it?" Contrast the form of
expression, Re 10:8. It is much more likely to be the cry of the
redeemed to the Redeemer, "Come" and deliver the groaning creature
from the bondage of corruption. Thus, Re 6:2 is an answer to the cry,
went (literally, "came") forth corresponding to "Come." "Come," says
Grotius, is the living creature's address to John, calling his earnest
attention. But it seems hard to see how "Come" by itself can mean
this. Compare the only other places in Revelation where it is used, Re
4:1; 22:17. If the four living creatures represent the four Gospels,
the "Come" will be their invitation to everyone (for it is not written
that they addressed John) to accept Christ's salvation while there is
time, as the opening of the seals marks a progressive step towards the
end (compare Re 22:17). Judgments are foretold as accompanying the
preaching of the Gospel as a witness to all nations (Re 14:6-11; Mt
24:6-14). Thus the invitation, "Come," here, is aptly parallel to Mt
24:14. The opening of the first four seals is followed by judgments
preparatory for His coming.
Similarly, the Cambridge commentary offers this:
Come and see The two last words are almost certainly spurious here and in Revelation 6:3; Revelation 6:5; Revelation 6:7 : the cry
is only “Come!” in all four cases. Who then is to come? Some say the
received reading, originally no doubt a gloss, is a correct gloss—the
Seer is to draw near. But the word is quite different from the “Come
hither” of Revelation 17:1, Revelation 21:9 : also there is no sign
that he does draw near or has need to do so, and if he has done so
once, why is he bidden to do it thrice again? Others take it to be a
summons to the Horseman who in fact does come: and this at least is in
harmony with the context, and makes good sense, and applies equally to
the opening of the first four seals where the same expression occurs.