Firstly, that assumes there would be no peacekeeping troops of any sort on the ground, which seems somewhat unlikely to me. But let's pretend that's the case.
If American companies are operating in Ukraine and using American workers then you can be sure America will protect them and any American government would react VERY strongly if American citizens were harmed. If Russia was to invade Ukraine it would be virtually impossible for them to guarantee the safety of those Americans or their - potentially very expensive - equipment. There have been plenty of instances of misdirected missiles hitting hospitals or schools and allied troops killed by "friendly fire" in Iraq, Afghanistan etc. So, to be sure they didn't kill an American and start a direct confrontation, the Russians would have to notify the Americans of their intentions and the Americans would then have to evacuate their people before any invasion began.
Franky, if you think the USA is untrustworthy enough to engage in that sort of behaviour then why would you rely on a treaty of mutual defence? What's to stop the USA simply refusing to honour the treaty?