1) I don't believe you can join tables from separate databases, but you could use data blending within Tableau in the way you need
2) Add to the level of detail - though this can add extra marks - there are usually ways around this with table calculations
3) Don't know, sorry
For #1, you could create an Access database that uses linked tables to link to your other Access databases. It's not terribly elegant, but I think it will work.
For #2, you may want to change any fields you add to Level of Detail from Dimension to Attribute.
For #3, I think you are asking about the View Data button in the tooltip. Go to Worksheet > Tooltip and uncheck the box labeled "Include command buttons". This turns off the buttons for Keep Only, Exclude, and View Data. There is no way to control individual buttons.