Geno boost does NOT stack with Quartz, unfortunately. If it was, that would be incredible.
My end game setup (post-Volcano) planned is:
Peach: Frying Pan, Super Suit, Troopa Pin
Mario: Lazy Shell, Work Pants, Attack Scarf
Bowser/Geno: (weapon), Work Pants, Quartz Charm
Every level up I get for Peach and Geno I'm putting into Attack. Bowser gets Attack except when the Special bonus is higher, Mario gets Special except when the Attack bonus is higher.
At level 21, here's what this party does, based on the damage calculations FAQ on GameFAQs, plus some of my own testing :
Against a goomba, Peach hits for 861, Geno for 663, Mario for 546. Total damage per turn = 2070.
Against, say, the Factory Chief (Defense 120, 1000 HP) and Gunyolk (Defense 130, 1500 HP), All three characters use Rock Candy on the first round, dealing 800 damage. Peach physically attacks Gunyolk, dealing 471 damage. Mario and Geno each deal over 300 damage, so they kill whichever enemy they pick. The fight is over in two rounds of combat.
Now, you had Mario only on level 16 by the time you entered the Gate. If we add Culex's experience to that, you're probably around level 18. Repeating the stars on Land's End once will be at least 19. I am also planning on using the lucky jewel to double the experience from many bosses. My goal is to be around level 20 by that time, which I may or may not be, because level 20 is the point at which increased level ups give diminishing returns in terms of stat boosts, however level 20's boosts give the greatest increases overall. I would like at least Peach to be at level 21 by that time, which is entirely possible since she's acquired last and therefore will start on the highest level.
Regardless of level, though, we also need to consider Rock Candy usage. Basically, with the Quartz Charm, I can do a minimum of 800 damage per turn to all enemies, from the beginning. Without the quartz charm, assume Peach has the troopa pin. Then we do 500 damage the first turn (Mario does 200, Peach does 300), 600 the second turn (Mario does 300, Peach does 300), and 900 each subsequent turn. In the long run it would probably pay off, but there aren't many enemies that have that much HP.
(Keep in mind, I am assuming unlimited Rock Candy usage since this is tool assisted)
Basically, I am about 70% convinced getting the Quartz charm is worth it, but it really depends on if I can convince myself it will save me enough rounds of combat on enough battles to justify spending a few minutes more in Monstro Town. And I'm not sure how much time all the saved time would add up to, and I'm not sure I will be able to figure it out unless I make two runs through.
Regarding killing Culex, I've killed Culex with the method I described more than once before, and although I was at higher levels, it could certainly be done at a lower levels because the integral part of it is the Super Suit/Attack scarf to megaboost Mario's magic attack stat, and using the Geno Beam the second time to shorten the number of jumps required.
