And yes retroactively - "You bind on earth" is present tense, "will be bound in heaven" is future tense - unless this is a translation artifact there is a clear causal ordering specified here.
Yes, and it refers not to changing what God wanted in the past (which would be retroactive, but isn't what a present tense action having future tense effects would indicate; it's not “whatever you bind on Earth will have been bound in Heaven”, which would plausibly describe a retroactive effect, or alternatively or pre-ordained one) but the future consequences of sin. Again, the quote you point to is the basis for doctrine about absolution of sin in confession, not the ecclesiastical infallibility either in general or in the specific form of papal infallibility, which are not closely related doctrines.
No, it doesn't. Either in theory (where the direction of effect is the reverse described and there is no retroactivity) or in practice (where the theoretical restrictions on the applicability of the doctrine mean that on any issue where there is otherwise any controversy, the doctrine doesn't force a resolution it just expands the debate to include whether any papal statements ever made on the issue met the criteria for infallibility.)
> Compare "It is not in heaven" with ""Whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven."
The latter isn't related to the basis for the doctrine of infallibility (whether papal or ecclesiastical) but it is part of the basis of doctrine around the priestly role in confession.