Which mons to immediately power up to L30?

C1: Just power to L29, the breakpoint against Mewtwo for 13 attack IV. It's better to find a Ttar raid, and pinap the Ttar.
C2: No. Evolve one of the karps.
C3: No. Find high level Exeggcutes on a sunny/windy day. Or Bulbasaurs on a sunny/cloudy day
C4. Hold it first.
C5: No. Just walk more Eevees for Espeon. I caught 5x 790+ Eevees in 2 hours on a partly cloudy day. And don't use the fast TM on Alakazam!
C6: No. That's a big investment just for one purpose. Even powering the Alakazam is better than this imo, especially now with battle parties.
C7: No. L20 Steelix sucks on defence, it's about on par with a L30 Cloyster. L20 Chansey does well on defence though.
C8: Up to you.

Fast TM your Blissey.

You don't actually need to power up anything extra for Groudon. Here in SG, you can just go to any raid before the egg hatches, get into the first lobby, just use 1 Eggy/Gary each and finish the raid.

You should definitely be able to solo Close Combat Machamp with 2 Espeons, 2 Eggys and 2 Dnites. Order:
Espeon C/FS
Espeon C/Pb
Eggy C/Psy
Eggy C/SoB
Dnite DT/Hu
Dnite DT/O

Contrary to DragoniteSlayer's advice, I'd say go ahead powerup your Zam for your Machamp solo team. Espeon's are nice and fairly reliable, but there's never a guarantee of the optimal moveset once it does evolve (though if you have plenty of eevee candy and high level eevees it won't hurt to try), and your Zam currently has it's functional best moveset (you want psycho cut for EPS and dodging since it's frail) and will for sure be more useful than an eggy without psychic.
Additionally, if you really think you need the help for groudon, and don't have a ton of extra candy to use to evolve more karps, powering and FTMing the gyary is not the worst idea since, unlike eggy and vaporeon, it takes neutral damage or outright resists all of groudon's charge moves.

Entei. It's going to be fire meta. Celebi regice registeel jirachi metagross. U don't really need to spend dust on a team for Groudon. Find level 34 35 egg in wild or other grass types against solarbeam. Use flying types against earthquake like dragonite gyrados and Moltres is actually good against. Water types against fireblast. Find level 34 35 eevee in wild and walk them for espeon for machamp solo. U can use 6 espeon as long as u dodge charge moves. Entei is going to be the best and most useful investment. Why stop at 30? Just cuz cp gain is half doesn't mean u should stop at 30. What if a breakpoint for a fast move is at 31 or 33? Charge moves scale better since they have high base damage.