I have more or less the same feelings as the OP and yeah, there is bearly any explination on whats going on politics wise etc. I had no idea where the First Order came from or why the New Republic and the Resistance weren’t part of the same team. It wasn’t until much later that I found it all out by reading. The New Republic didn’t see the First Order as a threat so started laying down weapons, Leia thought otherwise and created the Resistance etc. It’s a lot for a faneditor to cram into an opening crawl, especially when also having to explain about the map and Luke.
This will be a long post but my thoughts on how to do this was:
TFA - Somehow fit into the crawl who the First Order are and that Luke has gone to the island for the Jedi Temple. Luke is now needed but they don’t know where the temple is. The guy with the map at the start of TFA knows where and gives them the map (full map).
Events carry on as normal but Finn and Poe escape in the Tie, no crash. First Order go for the droid on the planet below as they got it out of him earlier. Next time we see Finn and Poe is when they enter the Resistance base together. Troopers catch Rey with the droid, the run to escape and board the falcon, Falcon vs Ties then Rey asks bb8 where the base is. Finn would have to be masked out of shots and any fire coming from the falcon will have to be removed as, obviously, Finn isn’t there to shoot. Will take a lot of work but Ive quickly looked over it and I think its possible.
Scenes from TLJ tell us that they are tracking the Falcon using the new tech. Rey lands at the Resistance base (can use shots from the planet where Maz’s castle is as the scenery is quite similar as far as I can remember). When the Falcon lands BB8 reunites with Poe and Han and Chewie enter (still a general and part of the Resistence) and discuss the map with Rey on board (whered you get this from? Jakku. I told you we should of searched there Chewie etc) BB8 shows the full map to Leia. Rey goes down the stairs to find Lukes lightsaber and leaves for training. First order appear above (as they were tracking the falcon) and they have to evacuate the base.
TLJ evacuation and escape is intercut with Rey meeting luke and training begins, Resistance lands on Crait. Film ends on Rey swinging her staff at the rocks and cuts on Lukes face.
TLJ - Crawl says that its been a few days since TFA, First Order know that the Resistance are trapped so have spent the time rearming and have brought in the walkers. Rey has been training for the same length of time. When we first see Rey she is now using her lightsaber on the rock to mirror her at the end of the last film.
Ive got tons more ideas and a good bit of TLJ worked out but I’ll leave it at that for now. I’ll be working on my own version at some point in the future (probably after EP9) but your welcome to take any ideas of mine.
Edit: Since reading your post again, the part about snoke - if it isn’t answered in EP9, I may shange the reflection Rey sees of herself to Snoke. In the waterfall shot after, instead of a voiceover from Rey, we hear Palpatine say something like “Darth Plegasis was so powerful, he was able to create life” or whatever he said. This would answer who he is and tie the sequels into the prequels a bit better. Currently, the sequel trilogy feels more like its been forced onto the end of episodes 1-6 and doesn’t quite connect as episode 6 seemed so final.