About Obi-Wan serving Bail... Don't forget that when Leia left that message with R2D2, Darth Vader and Imperial Troops had already boarded her ship. She knew she would not escape, this is why she sent the message off the ship with two droids in the first place. Basically, she was in a hurry and formed a short, succinct message getting straight to the point of what she needed.
If Plan A had worked and she had been able to deliver the message personally to Obi-Wan in the privacy of his home, I'm sure she would have had plenty of opportunity to elaborate on the exact nature of the relationship between Bail Organa and Obi-Wan Kenobi (and been introduced to Luke too). But as it was, she was under pressure and was trying to sum things up.
I sort of assumed it's a sort of power-sharing ploy between groups of merchant families/nobles/whatever political divisions they have on Naboo. Perhaps they've formed a co-operative council and share power, but are wary of giving anyone the top position? So, to stop them fighting over it, they select an adolescent from a minor family - a well-educated, unusually mature, "presentable" adolescent and he or she takes the role of king or queen, but being only a teenager naturally they take advice from the council. So any power struggles are about having influence over the king/queen rather than trying to knock each other off the top spot. They all remain equals when it comes to official power? And the monarch is just the "pure and perfect" face of government and the leader that everyone can love and invest in without getting bogged down in - or torn apart by - political rivalries or imbalances of power?
Or something.

That's my convulated attempt to explain it.
I still have trouble wrapping my mind around anything but a hereditary queen, but this is probably the best explanation I have ever heard. It actually makes sense for a Star Wars world.