First, I hope you enjoy your foray into firearms. We'd like this to be an enjoyable and welcoming experience for you.
That said, firearm ownership is eye-opening to the discriminatory liberal politics in California. Plus you explicitly ASKED about those politics. Being a part of a marginalized population will cause you to be "mugged by reality". So if this post diverges into that instead of introducing you to the shooting sports in a neutral manner.... now you know why. I will advise never voting Democrat. Ever. Proof is here: jump to 3:02:33 in the video where the Democrat-run committee is discussing tripling the tax on firearms "because guns are bad, mmkay?".
Rep: "Isn't this a like a voting tax?" -> Chairman (D): "lul k. who cares. vote aye." The flippancy they demonstrate in the video is eye opening and aggravating.
https://www.assembly.ca.gov/media/assembly-public-safety-part-2-20190409/video
Second, regarding your questions:
1. When a law is passed that will outlaw a certain weapon, do we always know about it beforehand? If I keep up with forums like this, will I always see when the axe is headed towards a certain gun?
No. Sometimes it is announced arbitrarily and on short notice. Sometimes existing statutes are creatively reinterpreted in "interesting" ways. Sometimes you are asked to fill in a form, and they retroactively redefine terms. Then confiscate or reject your form AFTER the submission deadline has passed. Then use this as justification for confiscation.
Generally, if the CADOJ feel like it, they will take your guns first and make you sort out the muck later. Read up on the history of "bullet buttons" and "featureless rifles" for a proven example from history. They arrested the same guy multiple times after the court found him innocent, and demanded the PD promise not to arrest him again, after the FIRST time.
However what the layers announce on the CRPA and NRA-ILA mailing lists is generally true. The CRPA is California-only, the ILA are the lawyers. Any notices of bills or laws go there first.
2. As far as we know currently, what guns are next on the chopping block? From what I've read on this forum, it seems to be ARs/Glocks?/SA Shotguns
Any and all semiautomatics. Starting with the scariest looking ones they saw their kids playing with in Call of Duty. (No, I'm not joking. Some laws Democrats have proposed in other states have fictitiously named guns [to avoid trademark issues] from video games.)
In 2013, a law was written, proposed, went through comittee, and passed both houses to ban ANY and ALL semiautomatic rifles as "assault weapons". The only reason it wasn't enacted was Jerry Brown vetoed it. They are certainly going to try again with "rubber stamp" Newsom in the same office.
3. What is the minimum necessary configuration of an ar15 that would allow it to be grandfathered in? Is it just the lower receiver? So, in theory, I could purchase 3 - 4 lower receivers to build out later, in case our rights are further revoked?
As it stands, a regular plain-jane semiautomatic rifle firing a 5.56 "military grade" catridge. But not a "civilian hunting" 223 cartridge. Or a "hunting" 308 cartridge. (As you get into firearms, you will understand why this is completely idiotic and highlights the disconnect between Democrat's proposed laws and reality... but it will happen. Mark my words.)
No, just the lower receivers are not enough. See my point in (1).
Against the written law, the CA DOJ started demanding people send photos of their guns to them. They then retroactively claimed a complete rifle was needed, and rejected submissions picturing just the lower the day after submissions closed. They also redefined certain legal terms, and rejected submissions for things as small as: missing a period mark, ticking a checkbox for a features they could-or-couldn't see, or thought were there but weren't. Generally acting like a bad mail-in-rebate house trying to deny as many claims as possible. (For reference, the first registration process in 1990 was mailing in a postcard.)
Long answer: buy anything and everything you want, or could foreseeably want, now. The ban will either come suddenly and cause a panic, or they will try boiling you like a frog in a pot.
First the "license tests", then the "fees", the ammo, then the gun. As an example of that: please try finding a store that stocks 5.7x28 ammo for the Five-Seven. Then refer to my point (2) about video games. They are now trying to pass bill to ban it from rumors and myths from video games.... when basic physics, demonstrations, and math prove otherwise.
TL;DR: Get an AR15, two spare lowers, (the roster limits your pistol choices severely) a "service pistol" 9mm [CZ75SP01, Beretta M9/92fs] or 45acp [CZ97BD, Rock Island Armory 1911], and a semiauto shotgun du-jure.
I do not list Glock since they are enablers of two-class-ctizenship LEO exemptions. I omit Springfield since they are overpriced for rifles, and although their 1911 are nice they actively lobbied for anti-gun legislation that would benefit themselves.