Hi @Mikeybell
Welcome to the Sonos Community!
As @Stanley_4 said, a Play:5 generation 1 cannot - and will never - be able to work on a S2 system. I agree with him - your best option (if the Play:5 hasn’t already been used for the Upgrade program) might be to buy a new speaker (the Sonos Five is the currently available equivalent) or set (you don’t have to get a particular speaker) and to sell the Play:5 gen 1.
Alexa will work with a Play:5 gen 1, but please note that it will only work with one Sonos system - if you had both a S1 and a S2 system, you could only link one system to Alexa reliably. So yes, you can control a Play:5 with Alexa, but only if the Alexa account is not linked to another system.
If all the rooms across the two systems have unique names and are on the same Sonos account and local network, Alexa may work with both. As mentioned, it may not be reliable, however. You could use Alexa with one system and Google Assistant with another.