I'll agree with you that Hogfather and TCOM left a lot to be desired[1], but I thought the Going Postal adaptation was very well done. Then again, I've done a fair bit of Discworld theatre (10 years ago) and seen many many of the plays, so I'm used to having subplots and characters dropped or modified, as it's necessary for any sort of adaptation to other media.
[1] I thought Hogfather & TCOM had brilliant sets and costumes and props that looked /just/ right, but the script, casting (not all characters, but quite a few) and direction was somewhat lacking. Thus I place the blame for my disappointment in those two purely on Vadim Jean (Writer/Director), whereas Going Postal was much better, with a different script writer & director.