It looks like xenon tetraoxide could be a candidate for your gas.
Xenon tetroxide is a chemical compound of xenon and oxygen with
molecular formula XeO4, remarkable for being a relatively stable
compound of a noble gas. It is a yellow crystalline solid that is
stable below −35.9 °C; above that temperature it is very prone to
exploding and decomposing into elemental xenon and oxygen (O2)
If you are up to making your world around this exotic chemical that would be cool. Or maybe if your chemistry is not that strong instead of Xe you could use some undiscovered noble gas of higher molecular weight than oganesson so you can assert undiscovered chemostry and not get called out by people who groove on calling out chemistry errors in works of fiction. They are out there, those folks.
I could imagine that a descending chemical rocket or a hot thing entering at orbital speeds might set off an explosion of all the accumulated substance C. Two strategies
1: Set it all off in advance. Drop some bombs, then after a minute or two follow them. Good stuff for an anime as the exploding green clouds of flame will be below your falling characters. The bombs will have detonated all available substance C in their path and it will be safe for your people. It might be kind of a mess on the ground after all the upblowing. The substance C will reaccumulate so no smoking once you are down.
2: Glide in easy. You people glide down from orbit, very slowly shedding velocity in their flying squirrel suits. Nice and easy, nice and easy, not stirring things up. It will take a while so they should have onboard music, or radio connection with their peers to play 20 questions.
thinking about coming down behind the bombs - there will be phenomenal updrafts of hot oxygen and xenon as the XeO4 dissociates. It might do a lot to slow down reentry.