I think up until (5) the argument has no errors, but all doctrines of Buddhism of which I am aware do in fact assert an infinite chain of causes. Below is Scotus's argument against the infinite regress:(1) No effect can produce itself.
(2) No effect can be produced by just nothing at all.
(3) A circle of causes is impossible.
(4) Therefore, an effect must be produced by something else. (from 1, 2, and 3)
(5) There is no infinite regress in an essentially ordered series of causes.
I think a huge problem here is the issue of "perfection" and "imperfection." Scotus might be using the term to mean rather "complete" and "incomplete" rather than "flawless" and "flawed," in which case (5a) seems to be the problem, and in fact it IS necessarily the case that a being possessing a causal power C possesses C in an imperfect (i.e. incomplete) way, if, per (5c) "dependence is a kind of imperfection." I believe Nagarjuna argued extensively against the notion supplied in (5a), but I am hoping to hear all of your thoughts!(5a) It is not necessarily the case that a being possessing a causal power C possesses C in an imperfect way.
(5b) Therefore, it is possible that C is possessed without imperfection by some item.
(5c) If it is not possible for any item to possess C without dependence on some prior item, then it is not possible that there is any item that possesses C without imperfection (since dependence is a kind of imperfection).
(5d) Therefore, it is possible that some item possesses C without dependence on some prior item. (from 5b and 5c by modus tollens)
(5e) Any item possessing C without dependence on some prior item is a first agent (i.e., an agent that is not subsequent to any prior causes in an essentially ordered series).
(5f) Therefore, it is possible that something is a first agent. (from 5d and 5e)
(5g) If it is possible that something is a first agent, something is a first agent. (For, by definition, if there were no first agent, there would be no cause that could bring it about, so it would not in fact be possible for there to be a first agent.)
(5h) Therefore, something is a first agent (i.e., an agent that is not subsequent to any prior causes in an essentially ordered series)
I know this post might be TLDR but I hope in any case that it is of interest!