Ps4 – put ps plus on wrong acount

No. Content licenses are bound to the account that activated them. Once the PSN+ account laps, it defaults to a standard PSN account, the free games that were licensed would then be invalid on that system for all accounts. But, any games you had actually purchased, even at a discount price would still be available for all accounts (unless you also removed the account from the system).

The only way to be able to play those games again would be to either renew your subscription to the account that originally downloaded them, or just buy them under the alternate account.

Specifically in regards to your question though. Any "free" games that you downloaded on your UK profile would have to be re-purchased by the US profile to re-aquire them. The only way you would be able to have them free for the US profile is if you had also had a paid US account concurrent to your UK account and had activated the game when (or if) it had been available for that account.

I know you may think this is stupid but remember, the free games are a perk for paid subscribers for that region. If you no longer subscribe to that region then you are no longer eligible for the perks.

Nope. You need to have purchased games to be able to play them, downloading a free game counts as purchasing. I know this because in the first few months I had PS+, I didn't download every free game every month, and when I later got back to those games to download them they weren't free anymore, if I had just bought them (for nothing) I would've been able to download them later, even when they weren't free anymore.

The reason you are allowed to play them now, is that your brother bought them and you have a secondary account on "his" playstation. This will change when it's no longer "his" playstation, but instead it's "your" playstation. Then he will be able to play your bought games, similar to how you can now play his. Hope that makes sense :P