Apologies if I seem harsh. I simply assume you want my honest opinion.
No. Absolutely not. By this logic, the bio-titans should have 2+ cover saves at all time.
No. If you have to assume that most people aren't going to like the idea when you post it, it's probably a bad idea.
There are alternatives.
1) make them run faster
2) give them stealth
3) shave a few points off
No one likes having their big centerpieces shot at by everything in the opponent's army. Tyranids are no different and shouldn't be any different.
The Trygon has 6 wounds, which takes a hell of a lot of firepower to bring down, and if you're deep striking right into a gun base, it's a suicide unit anyway.
Now, that said, I'm fine with allowing them to assault on the turn they arrive, but then you have to change Subterranean Assault so that it isn't so accurate, or make the Trygon 300 points, which I'm sure no one wants to do because it's a bad idea.
Fine.
No. No cover saves just for the sake of cover saves. Give them WS 4 and shave a point off.
No. People used to Fearless armies forget how powerful Morale can be and then forget how good Fearless is. Yes, you take more wounds, but maybe you should stop throwing unsupported units at the enemy and expecting victory or endless tarpitting.
Abused army lists and playstyles are not an excuse to further abuse the system. You should be out to make the codex solid and give a practiced player a chance at victory against any opponent with any list, not guarantee wins against most of the opponents you face.