Swanna is actually quite deadly if you set it up right. As St3ffo said, in the Rain, you have 100% accurate STAB Hurricane, access to Hydration, STAB + Rain Boosted water moves...Sure, it's not fantastic, but don't underestimate the nonsense Swanna can pull off. Not only that, but it learns Roost naturally! That's a pretty big checkmark in my book. Crobat to me outside of Mono Poison/Breeding is incredibly medicore. Your best Flying STAB is Air Slash, and that's not using your best stat, Cross Poison is relearn only, and Brave Bird is an egg move. On the other hand, it has quite a bit of utility in terms of Poison Fang for the chance of Badly Poisoned,. the fastest non-priority Confuse Ray in the game, and access to Rain Dance via TM(Praise be to Crobat the Charlotte neuterer in Mono Poison).
As for other things, definitely replace Emolga with Magneton/zone when you get the chance. Magnezone is approaching...soonish, but Magneton is solid in the meantime. Analytic hurts! As you also mentioned, Mawile can be a pain in the ass to retrain, but if you have a Sheer Force Mawile, absolutely go for it. Since the switchover to ORAS learnsets, you're actually REALLY close to Play Rough/Iron Head naturally. If you're willing to go to Level 45 for Iron Head, you can relearn Play Rough, or just tank it out till level 49. And if you're Level 37+, you have Sucker Punch, which is HUGE. It's a bit sad that you can't get Swords Dance onto it yet, but boy howdy, when you can, I will personally be trying to use Mawile. Anyways, cheers!
EDIT: Changed the stuff on Mawile since I remembered we're on ORAS movesets now. Thank you based Amethyst for blessing Mawile and other Pokemon with better level ups.