Potential weak area with DH for Merry to slip thru (advise please)

A quick update and then onto the needed advice! We have been NC for 3/4 of 2016. Merry & FIL have not attempted to conctact us at all, however i have had recent sightings near our home and a couple of drivebys (thanks video surveillance system)!! We've had FM's consistently and shes succeeded in her smear campaign of me (which I've handled with class and honesty of "not knowing what issues shes talking about, but she is up there in age.....her mind seems to be slipping"😉. One of the FM's recently received from DH a text from to not contact us again! The other is DH's sibling that has texted every couple weeks (at first pushing DH to see him & Merry & FIL) and now just random holiday hello's!

The advice part! DH has recently been talking about when FIL passes on (who will most likely leave this earth b4 Merry). Thanks to FOG, DH is worried about Merry's expectations on him to "take care" of her including maintaining her house (which is twice the size of ours and is mortgaged), managing her finances, driving her everywhere and pretty much everything FIL does!! Being NC with them has been blissful! DH mentioned that he will feel guilty for not stepping up or telling them so, but he really doesnt want to ever deal with her again!

We are aware Merry's expectations will be there regardless (she's a covert narcissist) but FIL has a level business head on his shoulders (for the most part but he's still snowed by his wife!!). Should we send a letter or let BIL arrange a coffee meeting? Our goal of this would be to layout the "we are not able to assist in anyway!" and lay the ground out to stand on later? DH feels he can stand that boundry later if he can convey it to them and his sibling now!

Thoughts/ideas/warnings? We would NOT be discussing anything else and will be 10 minutes or less in a public coffee shop (so I can record it....2 party consent state unless in public places!) I do not want to ever see Merry again, but if it will help DH's against her later....i can suck it up butter cup for 10 minutes and a 30 minute couples massage immediately following! As a side note....DS will be heading to my normal family members home......i wouldnt put it past them to surprise visit our home!