You only have to comply with the rules of the division when you're in the division. So Birmingham's spending this year has to comply with L1 rules, but if/when they're promoted they'll only have to comply with PSR rules in the Championship from next season. Ipswich were in a very good position when they came up despite spending a fortune in L1. Birmingham will be in a really good position
There's a rolling 3 year reporting period in the champ.
You can't just get yourself relegated, spend what you want for a season then come back with a blank slate.