Is there any point in powering up a Gengar with Hex?

I don't think so, Gengar only shines because of Shadow Claw (plus Shadow Ball), Hex is slow considering how fragile Gengar is, even has lower DPS than SC

About soloing Alakazam, I soloed him with 3 level 36 Tyranitar, a level 36.5 S/ FP Houndoom and a level 37 DT/ O Dragonite, the Zam had C/ FS so a team like that should do the job

You only really need the Shadow Claw Shadow Ball Gengar for the Psycho Cut+Focus Blast Alakazams if the dodge bug affects you (Tyranitar gets 1 shotted at max health so would not be usable).

You've seen my post soloing it, and I needed 3 Shadow Claw Shadow Ball Gengar. I doubt 1 would be of any use to you, but I was also living with the dodge but so Tyranitar wasn't really an option for me. Even then I had 2 Seconds left, wasn't the most convincing win even with 3 Gengar so I don't know if you can substitute Ttar in place of Gengar due to the (much lower) DPS moveset.

For all other movements it should be dealt with by 6 Tyranitar or Houndhoom, because the energy gain from Confusion or Shadow Ball/Psychic (undodged) should be sufficient to spam charge moves, but the pokemon all have to be powered past level 30.