I noticed in the first video [0:20] you seemed to miss the dash-attack by setting it too early.
It's an easy problem to fix but one that you can be really punished for.
I would simply say just wait that one extra second before pressing it.
Also the "predictable approach" problem is in the same category: easily fixed but easily punished.
Like Skrah said, maybe do an empty short hop for MGs, do a Nair instead of an Fair, or something similar where it's still a valid approach but throws off your opponent.
Also, first video, 0:48 and 1:37 you dropped DOWN onto a Fox.
That's a REALLY bad idea. I would say avoid this at all costs: they expect that and know how to punish you for it.
By the time you get down to them, they have a grab or a Bair waiting for you, trust me. Rather than drop straight down, drop over to the side and throw a needle or something.
0:59 the only thing I have to say is Grab the Edge! No matter really what position you're in, if you grab the edge and they're off the stage, you generally have the upper hand.
Why?
They can't grab the edge, so they MUST GO ABOVE YOU, or hit you to knock you off.
In this case, SCOTU would have had to go above you [as he was already], but seeing that, you can plan for it by placing a Nair or a Uair at his Estimated Position.
If someone's behind you but close to you, use a Nair because the sweet spot is Sheik's right knee...which is close to Sheik's body BUT behind her!
It's great for hitting [in particular] Link off the stage if he's below you, but I find it useful for anyone who's not far enough for a sweet spotted Bair.
Alright, I hope I helped you with your Sheik!
And just remember: practice beats skill.