These have probably been answered on here before but the search function is messing up for me.

1. Is the Fat Trout trailer park an entrance to the lodge?  Carl Rodd reacts strangely when he see's the old lady (forgive me, I can't remember the quote).  Agent Desmonds disappearance. And the electricity post that when seen is accompanied by Mike's sound.

the chalfonts lived there, two chalfonts to be exact, and with the ring on a mound of dirt undernenth the trailer, it is at least implied that magik/blacklodger stuff went on there. maybe that was the owl ring teresa wore. electricity is used frequently when a lodge spirit is close or involved.
deputy cliff lived in the park too, who later was to be shot by bobby and giggling laura in FWWM.

as Audrey said, the changing of backstory through the tv seris and film
was unavoidable. same location, different universes. twin peaks the show is not twin peaks:fire walk with me. and remember, lynch wrote or co-wrote only a handful of scripts, directed a handful of episodes (as he was starting 'wild at heart' at the time(but he stayed on as producer of TP)). there were so many other talented people writing and directing, especially the man behind the throne, mark frost. when lynch and frost were working together, they made flawless TV until about either episode 17 or 18, tho the mid-period TP show is better than what else was on TV. the time when lynch and frost kinda took a leave in the second season to work on lynchs wild at heart and frost's novel. dont get me wrong i love engels and dunham et. al. but there was a dramatic change of tone to the series until ep 24 or 25, when lynch and frost came bake to an extent (and then give Tv veiwers of ep.29, possibly the strangest disturbing and abstracted hour of television. the great final steach of episodes at the end are almost as good at the first season. and then he goes right into fire walk with me. it was hated at the time, but now revered as another great piece of film. just wild at heart.

2. The Chalfont/Tremond's grandson?  Lelend as a kid?

 well, found this link which addresses this one.

http://www.2000revue.com/community/topic.cfm?topicid=3982

yeah the theories are that he is either leland(or a manifestation of him, possible sexually), the magician of the series and of the 'fire walk with me" poem, or younger angent jefferies, as in his recolllecting of his expierience in the black lodge(again these scenes have pleanty of power cable superimposed

3. Donna going with Laura to the pink room and seeing how she acted and saw her with Jaques.  Wouldn't that have been helpful info during the series?

again, as audrey said, FWWM is a prequal. not in the traditional sense. sure
that would have moved/changed the plot around a bit, but this is (no offence) lazy criticism. films have to exist by their own rules. and from these each of our perspectives make it and form them. our world, and it is so so pointless to exists in nostagilia, even the nostagilia of now line of thinking. but i digress, FWWW was almost 4 years after the pilot. the red room scene did not exist as an idea when the pilot was written and shot. this show was obiviously open to changing and did so better, i think, than almost any television show made.