Good questions. While it could certainly work, having especially good synergy with her W Force of Will, I've found that Arcane Comet does not play towards Syndra's strengths as much as I would want her keystone to. Comet could certainly work in a Luden's game and you have a low range team, so poking the enemy is a priority. My main issue with the sorcery tree is that it gives neither supportive capabilities like domination's Zombie Ward and Relentless Hunter, nor the economical benefits of inspiration's Magical Footwear, First Strike, and Future's Market, all of which have inflated benefits for the low-economy support role. Manaflow Band is nice, but I personally view it as a crutch for poor mana management with how little benefit you actually get from it when not building tear items.

If you want to go Arcane Comet, go Manaflow Band, Absolute Focus, Scorch, and double adaptive shards to focus on lane harassment. It can definitely work, but it doesn't match my personal playstyle or goals for playing Syndra support. Vel'koz and Xerath (though his support has been nerfed over time) are usually better at being comet champions. Syndra will usually find herself all-inning far more than these other champions, dumping combos into stunned targets rather than sniping with one ability at a time. Electrocute, First Strike and Glacial Augment all support this playstyle more than Arcane Comet.

Melee botlanes are stressful but not disadvantageous for Syndra. She has some of the best disengage in the botlane, can harass them from range, and her Unleashed Power is usually a good counter to a melee botane's level 6 all-in, bursting one of them and usually allowing for a guaranteed 2 man stun and push. I've played against a few Pantheon Yasuo botlanes myself (for some reason) and spacing is the real key to success. Her hardest lanes are hook lanes by far, like pyke, thresh, blitz etc. A good pyke will just run you over repeatedly if you let him, although Syndra has some great options into him (canceling hook with E, pushing him away so his stun doesn't work, ult chases him into his invisibility).

Thanks, man I used to main Fiddle support back in the day, around season 6 or 7. You used to take E-W and when the enemy went in on you at level 2, you'd just throw the bird and succ the squishy. His E would hit the ADC 3 times in a row while you sucked, unironically doing 3/4 of their hp in a second if you took ability power runes and had thunderlords in masteries.