Hey @OndrejTrhan,
I tried those solutions before posting - but seems like the problem automatically resolved itself today. Today, the 1.5 version was not unavailable by default and had an “Install” button in the library. After clicking it, it did get back to “Unavailable” - but restarting the Epic Games launcher was sufficient to access the store page for it.
This is still not working super well - but at least, I have it!
Thanks,
Blaklis