Advice for a returning player

For the Chewie event I was actually going for Bossk, Jango, Boba, Cad Bane, IG88, as I haven't really worked on Dengar yet.

Dengar is actually crucial, and can get away with lower gear than the others. He prevents enemy stealth while he is under stealth. Since he should get hit right away by an AoE, this means that R2's first move will be wasted.

The problem is that if you can't prevent the stealth, then R2 can keep forcing you to attack people you don't want to attack. It's vital to kill Chewie first in that event... which you can't do if Chewie is stealthed by R2.

Now, it's possible to simply replay the event over and over and over until through RNG r2 decides not to stealth Chewbacca before you can kill him, but I think that's going to be farm more frustrating for you than just replacing IG-88 with Dengar. Fortunately Dengar is a Guild Store farm. You can get him pretty quickly, and since his role is just to sit there under stealth to prevent r2's stealth, you can get away with low gear. You might even get away with g8 on him, I'm not sure. He still needs enough health/prot to survive the AoEs that get thrown your way, but that's all. He doesn't even have to take a turn, although when you knock a few enemies out his Thermal detonator attack inflicts 5 detonators on random targets and becomes pretty deadly when they all (or most of them) land on one toon. When there are fewer enemies, that's more likely, so hold on to that attack for later in the battle. Other than that, his turns really don't matter.

Are they (Snow & Magma) better than Darth Sidious, for the time being?

I think so. Sid is so ridiculously squishy. He does throw out the debuts, but Snow gives empire allies TM when they kill someone (which is essentially what Sid will do, since the most likely moment for those debuffs to expire is when Vader kills someone) and if you load him up with Crit Chance mods he can use his AoE every turn, bypassing taunts and generally creating a lot of havoc.

Meanwhile Magma doesn't give TM, but he takes TM away from enemies with his AoE.

The strength of the Palp squad is TM manipulation and both Magma and Snow provide methods to do that, so they'll both contribute to the core power of a Palp-led team. Ideally they would also throw more debuffs around, but since Sidius dies so quickly, they're ultimately more useful than he is.

That said, your Magma is only g7 right now, so it's possible that Sid can be useful for another week or two. I just wouldn't invest in him. The other two toons have more potential starting a couple weeks from now when you get one or both to g9, and in the longer run Bastila (Fallen) is insanely better than Sid.

By the time you ave to give up BF to your Revan squad you'll have even more choices for the Palp squad. Range Trooper (for protection recovery), Piett (for insane synergy with Vader), Gideon (more TM manipulation + debuffs) are all at least better than what you have now, and Piett is even better than BF on a Palp team.

Sid today, Snow/Magma soon, BF in 6 to 10 weeks, is a good general strategy, with lots of time to find a good option to replace BF by the time she leaves the squad.

I already have a (low gear) Raid Han.

You can hang back on his gear right now, but remember that after you get to the point where you have a zeta to spare for him, getting a first shot off with automatic stun can be incredibly useful even if you don't have your CLS squad together by that point. He's not entirely plug-and-play, but there are times when getting that first stun is worth throwing him in to a non-synergy squad even if RH did nothing else the whole battle.

I'm not saying prioritize his gear or his zeta because a functioning Empire squad and an improved Arena squad are both more important, but he should definitely be zeta'd soon after those priorities are taken care of (Bossk is another strong candidate for next zeta after GR &Empire).