I would say, yes, the original sentence is awkward, but given the current trend towards the minimal use of commas, it would certainly be 'acceptable,' where acceptable means 'not necessarily incorrect.'

The first correction works just fine. However, I would probably choose to offset "later with Fran Tarkenton" as a parenthetical remark in commas. Either way, or even moving "later" to the first of the sentence, I think a decent argument is made for retaining "co-founder" because, technically, the base sentence should be treated in grammar and diction as though the parenthetical remark does not exist.

I agree that the second correction is the best and most efficient, and that it does not require the use of "co-founded" because "founded" as the verb of the compound subject does in fact make the case perfectly clear -- to the extent that these two were the only two founders. If there were seven founders and the point of the larger narrative is to cover a particular relationship with Fran Tarkenton, then treating both of them, together or separate, as "co-founders" might be the more accurate thing to do. (I'm not sure. I can see how "co-founders vs. founders" is probably a distinction without a difference in most and perhaps even in all cases. I'm actually quite comfortable with that point of view.)

Notice, also, that there is a choice here in emphasis as well as word economy and rhythm as to whether Fran Tarkenton's involvement is a) parenthetical or b) an explicit part of the sentence. This difference is, to be sure, subtle, but it is a consideration that is certainly of equal or greater weight than the subtlety of where to place the adverb "later."

By the way, most editors of technical writing and 'international English' (whatever that may actually be) agree that qualifiers such as "later" are, in fact, best placed at the beginning of the sentence.