Gather strength, Pick your MomentIf you let DH write the letter and send it, then you still have the option to send your own letter in the future, which would then have more impact than if DH never sent his letter and you opt to send your own letter first (2 letters vs. 1). And, if you send your own letter first, then events may unfold and DH might then refuse to write his letter later. So, do what you have to do to have DH write the letter first... and hold your cards.
HOWEVER, that said, I think holding off on this issue totally, until the next visit, makes sense. Then, any issues that come up next visit will be fresh and usable as examples. You can deal with it in the moment then and it will have more impact. You're right, the words, spoken or written, may be forgotten by the next visit. You also have time to reflect and in a few months you may have new ideas on how to handle it, also gain your fortitude and your peace about how to deal with it, instead of being reactionary in any way. I'm NOT saying to back off at all... just delay. Pick your moment.
We had an uncle in our family who always horseplayed with us at Christmas... the one time of year we saw him. He never grabbed butts. But, even so, it felt wrong. I always wished our parents had stopped it.
"I want this to be mentioned one time and then the problem be solved."
--Maybe this is asking too much. (?)
"I do want to handle this situation in a way that grandpa keeps his dignity"
--you might have to keep reminding him more gently in order to accomplish this(?)
"I'm considering contacting his brother to find out his opinion."
--I probably wouldn't do that. That might be inviting a feudal situation or asking to take sides, etc. I would ask for information, maybe, probably in a more covert way. I wouldn't get into discusssing it at length with other people of DH's family unless I was very close to them. Exception: BIL has kids? Maybe HIS wife has issues too? Maybe BIL would then have reason to be involved in the situation (ie. some kind of family "intervention"?)
"He doesn't feel it's necessary to call up FIL and discuss this now. We are home and there's 2500 miles between us. I'm just still reeling from the trip and concerned about ALL future visits with his family. I do feel scared and a bit powerless as well."
--I agree with DH on the timing. See above. You're Mama Bear emotions are up on this one. Take a breather. Your boys are safe right now! You can't pounce on a predator unless they're close enough. Find your own place of peace and power before deciding what to do.
"What if we got divorced and I wasn't there to stop this?"
--Is divorce an option for you right now? Not good to entertain this kind of thought. I understand the "what if I died" question and I'm sure that would have different solutions than the divorce question. Many of us, I'm sure, have passing thoughts that parenting on our own would be easier... but, that's usually not true. We're just stressed and having a hard time dealing with the current problem. It generally would NOT be easier to be a single mom. This question or thought is just clouding your judgment at this time! You and your DH are generally a united front, right? So, hunker back into the security of your marriage, find your strength there, and don't act until a moment when the marriage is acting out of a position of strength. You will have much greater impact when you are together on this issue, or any issue. Again, you have time to cultivate all of these.
Please see my original post for some great references on NonViolent Communication.