Wrzlprmft,
Wow... that's a lot of ground to cover but I'll do my best.
Deities are a concrete fact in Yliakum, since they appear directly in front of the citizens from time to time and use their powers (GM events), so you could say that religion is present and pervasive from the moment you enter the world. HOWEVER, just because they exist and have clearly demonstrated powers of godhood does not mean that everyone must pick a side & pitch in to help that side as if they were lining up to play dodgeball in school. (Small attempt at humor there.)
Magic is most definitely of divine origin. Talad created all the Glyphs that exist in Yliakum, and they are the only current source of magic for the denizens of this realm. Thus the glyphs are what limit the citizens' magical abilities. There is no known spell that can be used to force another to follow a particular god.
The gods are interested in being worshipped, as evidenced by either their specific requests to have temples built in their honor, the fact that they don't blast to smithereens temples they didn't request, or their occasional appearances where they directly ask for fealty.
Worship can bring with it items, knowledge, training, and other goodies, either through quests or GM events. However, godhood does not equate to absence of spite, jealousy, anger, etc., so some gods would not be pleased to find a citizen hedging their bets with another god. Also, you will find that some gods are polar opposites regarding goals, desires, and worldviews, so it might be hard for a person to follow two at once with complete honesty. Regarding the issue of serving a god dishonestly... it could get messy but it's your choice!
I hope I've helped answer some of your questions.