You can be over the cap by 10%. That is sitting a different cap for part of the league
year, not eliminating the cap. No reason that can't happen in the playoffs.
None of the rest of your post matters, they can literally do whatever they want to enforce a cap in the playoffs as long as it's negotiated with the NHLPA.
You are stuck on a formula that is only used part of the league year already.
They can create a new clause that deals with the time period between end of the regular season and June 30th. Just like they have a different accounting July 1st to the end of trying camp.
I think you forgot something from that clause in article 50.
"League year"
Stanley Cup Playoffs =/= League, Although it can happen during the said league year, it is still not the league.
This is back to technicalities. Although NHL own Stanley Cup, Stanley Cup is not assimilated with NHL since originally, NHL bought the Stanley Cup rights.
If Stanley Cup Playoffs are the league, Why not call it NHL Playoffs like NBA Playoffs? Why in the pandemic year that extra round is called NHL playoffs and needed to be separated from Stanley Cup Playoffs?
Terms NHL champions never exists in any history of books, only Stanley Cup champions (Unlike NBA champs).
Article 28 said players and clubs still got paid in playoffs but only a playoff pool money (which is basically prize money accordingly where you finish in playoffs), Plus, the presidents trophy winner also got some prize money. But remember, this is NHL who gives it. Not the club that where the players work. So different cases.
Even though they get paid from 1 July to 30 June the next year, the annual salary calculated based on "SEASON DAYS" which most likely fixed (unless that pandemic and extensive lockout happen).
You may say they can change it but how? Plus, you gotta maintain those two entities that are similar but not same (NHL and Stanley Cup Playoffs/Finals).