Hmmm so that's some interesting questons. Let's see.
How did Chomper escape island?
Chomper escaping the island could probably be easily enough explained with Elsie. Chomper's parents on the other hand...I don't know how they would ever escape.
Where is his parents?
Like I said, I can't imagine a way for them getting off the island but I honestly wouldn't even care if they were just somehow not on the island with no explanation.
Why Redclaw is so terrify so group of Mysterious Beyond residents came with plan to use Chomper (with Ruby's help) to learn life of Great Valley residents, and somehow defend themselves from this sharptooth?
I'm not so sure it was a whole group decision that came up with the idea. Since the plan is to observe and find out how Great Valley dinosaurs work together then I imagine that most groups and herds are isolated and separated from each other. Larger herds like Bron's or other farwalkers that have visited the Great Valley are not likely targets for Red Claw due to their large sizes so they are not likely bothered enough to care.
So what you end up with is a lot of smaller groups like Ruby's that don't know how to deal with him and his unorthodox method of hunting with Screech and Thud. I'm pretty sure that this idea originated with Chomper with having experience with teamwork with different species through the gang.
How did Chomper and Ruby met?
I feel like Chomper would have to somehow get separated from his parents for his initial meeting with Ruby. Maybe Ruby does something to save Chomper since we know his parents become fairly accepting when that happens. Red Claw could be the initial cause which would lead to Chomper's idea as well.
Why did Ruby's family not go to the valley, when they had two baby hatchlings?
Probably because they're oviraptors and they feel like they wouldn't be accepted into the Valley, especially a whole family of them. With just Ruby along with Chomper there might be enough leeway given for them to live in the Valley and with the apparent importance of Chomper learning how the Great Valley residents get along.
Please, tell us how you see good version of prequel movie to events of LBT TV series in your head! 
So a sort of breakdown of events based on the questions above:
-Chomper and his parents get attacked by Red Claw, Chomper gets separated.
-Chomper meets Ruby and her family
-Ruby and her parents tells Chomper about Red Claw's uprising after Chomper describes what happens
-At some point Chomper reunties with his parents, he tells them of his plan and they agree to it and let Ruby go with him
-Exit Chomper's parents for reasons?
-Ruby's starday/ Ruby and Chomper head towards Great Valley
-Maybe throw in some sort of escape from Red Claw on way to Great Valley to serve as a climax.
Honestly, I feel the most important parts could be done well enough within an episode duration and wouldn't need a whole movie dedicated to it.
Maybe I'll do my fanart prompt based on something here. That might be fun to do. Thanks for the question and for the potential idea.
