TO answer your questions.
Weird laws in the US have loopholes for Sportsbooks. These loopholes seem to stem from allowing Horse Race Betting online. The laws are very convoluted tho. It may be a para mutual thing, which pools bets from players against other players with the house taking a rake. Sound familiar?
As for the credit card thing. UIGEA sets up a situation where money to gambling sites is the problem. Gambling is loosely defined (these days) as a bet against the house. But the laws are poorly defined.
While Merge has made some very peculiar moves recently, they still offer the best game selection and a pretty comfortable table to play at. No reason to believe they will fold other than thinking 'they ain't what they used to be', which is largely true.
The Cake network isn't isn't called Cake anymore, it is now the Revolution network. Cake itself got out of the US, and transferred our accounts and money to Juicy Stakes
. Same international player pool.
Bovada was Bodog when you left. With their Sportsbook, they have maneuvered themselves into a unique position that they believe will be fine with existing US laws.
Sportsbook in particular uses poker from Merge. In fact, the current cashier at Merge (Jazzette) comes from the company that owns the Sportbook website.
These are general answer to general questions. IANAL, but over the years have followed these issues fairly deeply.
Personally, if you treat it all with an entertainment POV, you can only lose another ticket to an evening of entertainment. If you treat it as a biz, well, you need to be up on the twists and turns of the industry as a whole, and the legalities in particular.