No, it installs like the regular plugin...  each network site (and the man site) are standalone...  after installing you can see the directories used on forum - integration - storage location...  you can also customize those...

there are some intricacies of multisite that are not completely understood - likely because none of us here actually use it...  when a user registers on a network site, it should create the account and all - in fact it does in testing... but there have been a couple reports similar to yours where the account is created, but simple press never gets notice of the account so doesnt create our added member info... and as you notice, we have a fallback in place so if the user logs in and his added member info has not been set up, we do it then...  this is rare, but has occurred before... we just have not been able to duplicate it...  must be the manner in which the registration occurs - such as a plugin that does not use the wp api..

as to welcome email, are you sure forum - components - login and registration, and forum - options - email settings are configured properly on the network site?

are you sure the users are physically registering on the network site?  and not on the main site signup with some some of membership plugin that copies the main site user down to the network site?