I think OMT definitely wanted to make clear to Ennis that he knew about his relationship with Jack.. and he wanted to rub in the facts that (1) Ennis didn't know all there was to know about Jack (i.e. "the other fellow" and the deliberately ambiguous "most of Jack's ideas") and (2) that OMT did know a lot about Jack... more than Ennis expected. In some ways, I think OMT is trying to make Ennis feel guilty about never coming to help on the ranch.
So, yes, I think OMT is being snide, for sure.
But, I think Lureen is very benevolent on the phone. Lureen, I feel, is extremely kind to Ennis and it's thanks to her encouragement that Ennis proceeds to go to Lightning Flat and thus find the shirts, etc. The nicest thing Lureen does on the phone is to let Ennis know that Jack said Brokeback was his favorite place. She didn't need to say that (especially given the idea that she's just now having a realization about who exactly Ennis is), but she does. And, I think it shows a kind side to her.
I don't really know what she means about other possible wishes that Jack may have had (beyond the ashes). But, I don't think she's at all meaning to be snide. I think, like OMT's comment, it simply goes to show that Ennis doesn't know Jack so well in certain aspects of Jack's daily/family life.