Hopefully there's a good reason that on-one has does this in canon yet
Since heating the myomer will still heat the 'Mech, it being an airtight shell and all?
As for the Clans, they're well aware of TSM-X. Was just rereading the Blood of Kerensky trilogy, and what happened was Focht brought up the TSM-X with Myndo, who started reminding him of all the pitfalls of which he was well aware, Focht went on to explain how it could still be a nasty surprise for the Clans, at which point Myndo ordered him to tell the Clans about the gas and instruct them in its use, to which Focht replied he'd already done that, but this was all beside the point, since he was trying to impress upon her that ComStar has no idea what the research status is on a FedCom project that's twenty years old, who knows what they've got in the works now and what threats that might pose to Myndo's grand vision for the Clans, ComStar, and the InnerSphere.
Kai's 'Mech mounted the "perfected" TSM, the one that requires running at 9 heat for best results. Kai complained that this would "cook his targeting circuits a little," so yes, 'Mechs with TSM need MechWarriors trained in how to get the most out of it and compensate for the downsides.
As for why the FedCom didn't think to equip a bunch of 'Mechs with TSM-X during the Invasion: well, they were running as fast as they could just to stay in the same place. The Clans were gobbling up worlds faster than the FedCom could reinforce them, and refitting 'Mechs with a twenty-year-old technology that might provide an edge against only that enemy wouldn't be appealing when the same resources could be applied to repairing damaged 'Mechs or outfitting whole new units or retrofitting old ones with other, more useful recovered/developed technologies. Plus, since Yen-lo-wang got the "refined" TSM in 3051, it means TSM was almost or actually ready for deployment when the Clans invaded, no need to use the old, bad stuff when we've got the new, good stuff.