I haven't posted to you before, so sorry if I'm sticking my nose in...
It seems to me that the funeral is about celebrating and honoring the life of "Uncle XXX," and being there for his family, NOT about whatever drama may unfold between STBX and the in-laws.
If the family of "Uncle XXX" has expressed a desire for you to be there, and you want to be there for them, then GO.
YOU have done NOTHING wrong. If STBX wants to create unnecessary drama by bringing the OW who didn't even know "Uncle XXX", it's on his head, not yours. He will be the one who is looked at and pitied as a fool, not you. And that will happen whether you are there or not.
Your FIL showed great courage and strength (IMO) in standing up to your STBX by supporting your right to be at the funeral. Your MIL is running scared.
I understand your wish not to cause MIL anymore pain, and you are the only one who can ultimately decide how to handle this and what to do, but....this is what your STBX is choosing, not you.
If a D does become final, would you let your MIL forever dictate what weddings or funerals or baby showers you could or could not attend?
I say again, if the family of "Uncle XXX" has expressed a desire for you to be there, and you want to be there for them, then GO.
This is about the life of "Uncle XXX," about honoring his memory and supporting his family, NOT about whatever drama your STBX is inflicting on everyone else.
Just my 2 cents.
Me:40, xH:41 M:19 T:21 D14, S10, D6 IDLYA bomb:12/22/06 OW bomb (21 yr. old employee):12/23/06 H move out 2/07, OW move in 5/07 D papers served 6/07 D final Nov. 26, 08 :-(