Quite a few FFPs allow retroactive credit of miles for flights flown up to a reasonable period of time before the commencement of membership. I know for a fact that Krisflyer allows flights flown in the 6 months prior to joining the program to be credited. [Claim form here if you happen to take an interest in Krisflyer:
https://www.singaporeair.com/saa/en_UK/docs/krisflyer/Customer_Service/KFRetroActiveMileageClaimForm_Dec06.pdf]
Best to compare the FFPs with the carriers/fare classes you were booked in and projected earn/spend ratio therein before banking your miles IMHO.
AC, UA, US, NH and NZ offer paid lounge memberships however I have no idea what other lounges you're able to access other than the lounges of the respective carriers. Alternatively, AC 's Star Alliance Gold tier ('Aeroplan Elite') can be achieved with 35 000 status miles and allows access to all designated Star Alliance Gold lounges.
Now paging the *A experts....
