@Zachary Freeman
1. Based on our experience in LA City, I think that AB 2097 would apply whenever you are adding units (whether that be through alteration, addition, or new construction). However, you need to make sure that the property in question really does sit within a 1/2 mile of public transit, per whatever definition Inglewood has set for public transit.
I don't see a clear definition from the state on what constitutes public transit, which makes this tricky. Here's an example from LA City showing why it's tricky: In LA City, for AB 2097, the LADBS determines "public transit" using their Transit Oriented Communities map, which has a more convoluted way of determining what counts as a "public transit" stop. However, for ADU parking requirements, they just let measure 1/2 mile to any sort of transit stop. The former applies to far less properties than the latter.
2. I would love to be wrong on this, but my understanding is that you can't grandfather any nonconforming condition that has already been remedied to make it conforming.
In terms of the ADU, I'm reading Inglewood's guidance and it seems to contradict the state law. In short, the state says you should be able to build 2 detached units.
The state laws say that a multifamily property can have 2 detached ADUs, regardless of the unit count on the site. The 25% limit only applies to conversions of portions of multifam structures not used as livable space. (See page 20 of the 2022 HCD ADU Handbook and 65852.2(e)(1)(D) of the CA Gov Code)
Inglewood's "ADU/JADU Development Standards" PDF says that the 25% rule applies to detached ADUs...
So, Inglewood might put up a bit of a fight, but I don't think they can stop you from building, say, a detached duplex w/ 2 ADUs, especially if each ADU is <800 sq ft.