I wouldn't really say it's necessary to power up as a defender but personally I would simply because I like having Exeggutors towards the top of my team and the candies come fairly easily to me. Unfortunately Exeggutors only really get around 2400-2500 CP now, so if that would be enough to hold a higher spot in a gym where you are, I'd say it could be worth it
Exeggutor Confusion/Psychic
Still a good mon to spend candies and dust? after evolving 4 for offense i got this one 91.1% IVs Confusion/Psychic. I guess he would be welcome to my defensive team, there is no Grass/Psychic Pokémon:
Dragonite 3271CP, SW/DC;
Snorlax 3027CP, ZH/BS;
Dragonite 2867CP, SW/DP;
Dragonite 2826CP, SW/HB;
Lapras 2617CP, IS/Bliz;
Vaporeon 2554CP, WG/WP.

Answers
I would. I have the same one.
Confusion is also better for attacking if you dodge quick attacks since you can only use zen head butt and confusion once between having to dodge.
I powered mine up bcz it doubles as a good attacker as well.
Solar beam take too long and I normally get hit by 2 quick attacks or even a charge mid-use. Psychic on the other hand you can use right after the other mons charge attack and still dodge their next quick attack
You have Cooldown+1.5~2.5s between AI quick attacks (check it here https://pokemongo.gamepress.gg/gym-combat-mechanics). We can consider cooldown+2s, so any 0.5s cooldown Quick Attack will take 2.5s by AI. You can use Zen Headbutt two times 24damage/~2.1s or a single Confusion 15damage/~1.5s before dodge. Thats why you will see Zen/Solar Beam rated A and Confusion/Solar Beam rated B for attacking. Thanks!.
I'd say yes. One of my favorite defenders. I have a 2635 CP Exeggutor with that moveset and it has the highest 'battles won' count for me as well (6). I think it's doing so well because of the lack of fire Pokémon around my town. Anyway, I don't think that placing highest up on gym is that important as many people seem think. Yeah, I understand that it has higher chance of staying in a gym but here where I live the gyms rarely get to level 10 nowadays and if they do, they get taken down really fast, doesn't matter if your Pokémon is the first or the last. This is just my personal opinion and I like to have different defenders rather than just 10 Dragonites and Snorlaxes.
One of my favourite defenders despite his weaknesses. I've powered mine up to 2500 and it's pretty good. I like the fact that I can prestige it with my parasect army. I tend to take over a gym, plant him and prestige a couple of extra spots (there's always one idiot who'll drop a Rat or Pigeon in just for the dust, but generally I'll find you get a couple of decent Pokes put in below you).