I have a 100% vanilla world currently in 1.8 that I'm most likely going to switch to forge for 1.9/1.10 in order to disable the weapon cooldown with a mod.

 

My question is what kind of sideeffects are possible should I decide to switch from forge back to vanilla again? Obviously any new blocks/items related to the mod would be removed automatically. Is there anything beyond this that could result in world corruption/loss?

Yep. World's been vanilla for a couple years.

 

If I DO have to go forge no mods, are there any known occurrences where some mechanics might work slightly different than they would in vanilla? Pretty sure the answer is no but the voice in my head says to double check.

Best thing to do would be simply [glow=green,2,300]backup the map and try it out[/glow]. If it doesn't work or you're not happy with the aftermath, restore the  map from backup onto your initial version. :-)

 

I've had times where mods just instantly break, For example when using carpenters blocks, the air block was glitching and replaced with carpenters blocks, Logs with Mail boxes and grass with some sort of liquid.

I'll definitely backup. My worry is more after putting another dozen hours plus into the world. But it looks like I'll be all good