just hoping that they don’t take anyone else out with them
This is a really important point. The fact they still drive is ofc a high risk vector for doing just that. Goodness it's tricky!
It's my own aspiration not to "take anyone else out with them", which is why my shortlist of ways to kill myself once i eventually find the courage to do so, does not include gassing myself in my home. It would be simply indecent & immoral if my action, post-cark, caused harm or death to others by the gas-filled house finding some ignition source i'd overlooked & exploding.
In your OP you said...
but once I do notice, I want to confront and solve it
...which is an understandable & IMO admirable trait. Given all that you've generously shared with us in this thread, it seems highly improbable that you will be able to do so in this case, no disrespect intended, given...
denial & minimisation ... blithe denial
...& so it would appear that your...
still be there for moral & financial support if necessary
...is perhaps prolly the least-worst outcome available, & thus i suppose your challenge/burden is to find some way to reconcile yourselves to it, despite your druthers, & knowing that emotional pain lies ahead in the near-term.