Multiple Site Login Question

Please do not take the title of my post as Multi-Site, I’ve tried Multi-Site and had more problems than I want.

Here is what I am trying to do:

Lets say I have three websites, for ease of understanding, SiteA.com SiteB.com SiteC.com, all currently under their own installs for wordpress.

SiteA.com is the main site which also contains a Xenforo install, I’ve found an incrdible WordPress/Xenforo bridge which works wonders for the current site plan.

I want SiteB and SiteC to point to the forum setup on SiteA. Meaning if a user logs into SiteB or SiteC he will also log into SiteA, the forum controls the user accounts and shares them back to the WordPress database.

After much searching through the forum I believe if I change the salt settings to match the ones at SiteA and adjust the wp-config file for a cookie_hash it should take care of what I want it to.

Is there an easy way for me to merge the databases of all three sites with users already established?

Is there a widget of some kind floating about in the world, that I can point a user’s login to another site. User goes to SiteC.com and enters his username and password which actually logs him into SiteA.com?

End goal, a network of sites brought together by a community, which will allow users to comment on posts or discuss the post in the forum.

Hope you got all that, I tend to get lost there as well. And I hope you can all help.