Heimerdinger, Academy Instructor. For the questions, assume my Heimerdinger is at a battlefield causing the replacement option to be active. Keeper of Masks - When you play me, play two Reflection unit tokens here. They become copies of me. Does this mean the reflection is replaced with a mech and then it becomes a copy of Keeper of Masks? End state being 2 Keeper of Masks cards?

Yes — each Reflection is replaced by a 3 Might Mech token, and then Keeper's follow-up "then do this: They become copies of me" applies to the resulting tokens, so you end with two Keeper of Masks token copies (plus the original Keeper, 3 units total).

Rule 375 is exactly on point here: a replacement effect inherits modifications from a linked ability referencing the replaced event (its own example is a "play a 3 [M] Mech token. Then do this: Give it Temporary" being swapped for a Recruit, which still gets the Temporary). So the copy effect doesn't get skipped — the Mech becomes a copy of Keeper, overwriting its 3 Might and Mech tag with Keeper's 1 Might and rules text (Rule 479.1.b). Heimerdinger has no once-per-turn limit, so both tokens can be swapped, and you choose per token.

Two corrections: these are tokens, not cards (Rule 185 — Keeper's copies keep their token nature even while copying), and there's no infinite loop — the copies gain Keeper's "When you play me" text, but they were never played, so no new Reflecions are made. Note the swap is actually cosmetic here: if you decline Heimerdinger, the two Reflecions become Keeper copies anyway, so the end state is identical.