Keep getting email adds from Rainer Arms for their ambi billet lowers. They are overly priced and unnecessary IMO but the thought came to mind, are they even legal here with the ambi mag release?

Pretty sure Thorsdon makes a CA legal ambi setup.

Oh and to your question, if you can drop a mag without a tool it's illegal unless it's on a featureless.

Found it. LINK

The thing I do not understand is what's the difference between the two Thordsen mag locks - UBMR and UBML?

I've wanted to buy the installation jigs and mag lock mechanisms but, honestly, I'm not sure which to order.

UBML is for bullet button rifles, UBMR is for normal rifles

Yea they're legal. Three ways to do it.
1) Use it with a rimfire upper. LIke a spikes, cmmg, etc with a 22 upper and you can use the lower as is
2) Bullet buttoned - bullet button required and usually they disable the ambi mag release feature. Ive seen them do it on lots of other ambi lowers with dual mag releases.
3) Featureless - Pin the collapsing stock (no idea how you would on the wire stock but there's always a way) and then put a  grip wrap around the pistol grip. Yea it's horrible but hey, i did it for a scar heavy, a XCR-m, and others. This allows you to retain all ambi features, which is actually benefical for a grip wrap. Mag release, selector, and bolt release all on both sides! And you can use your 30 rounders as well.