#2 - APPLY VASELINE!! Put COPIOUS amounts on the flat rim where the cap touches the base, and also on the O-ring all the way around. Put some on the threads too. This will help reduce the friction and get that seal to work. You shouldn't have leakage. Is your downpipe touching the floor of the base and keeping the O-ring from seating? If so you might want to trim it up a bit. In any case, keep in mind that some day you're going to want to get that base off and things you do now will greatly facilitate it.
Vaseline? I know Vaseline eats rubber like it's going out of style (depending on the rubber, of course). Perhaps a food safe silicone based grease would be better? I'll have to take it apart next weekend.