About Temporal Breach, when I banish a friendly unit, must I play it immediately or can I wait to play it anytime?

No — the play-back is immediate, part of Temporal Breach's resolution. You can't bank the banished unit and play it at a later time.

Temporal Breach reads "Banish a unit, then its owner plays it to the same location, ignoring its cost." The "then" instruction executes right after the banish during that same resolution — the unit goes straight from Banishment to pending on the Chain and is finalized to the same location (entering exhausted, unless you pay Accelerate). There's no point where the unit sits in your Banishment waiting for you to choose to play it later.

The only way the play-back doesn't happen is if the instruction is impossible — e.g., the location can't accept units (Rockfall Path), the unit is a token (it ceases to exist in Banishment), or a mandatory additional cost can't be paid. In those cases the instruction is ignored and the unit stays banished (Rule 359.3.e.6); you still don't get to play it later.