You need to work on your killing moves too. You got him up way too high on halberd. Samus can be very powerful if you use her good. Learn her dtilt. You can get easy kills when they are at a high percent and try to attack you up close.
The best time i use dtilt is if they are coming at me in the air, i throw up a shield, and they hit it. Unless the move they just can be canceled by landing (Like samus's missiles in melee) they are a sitting duck waiting to be roofed. Learn simple techniques, like baiting someone to hit you.
For example, taking dtilt again, people can hardly stand letting samus sit there charging up. I wish i had some way to get a video online but i can't, because i have a perfect example of this on one of my replays. I was able to push my opponent (pikachu) slightly away from me and in the air. I then started charging up to just after he regained control of his character, then threw up my shield. He went straight for me when i started charging and hit my shield, then i use a dtilt to put him in the same position. Did the exact same technique and it work again, until he decided to retreat and i released the shot knocking him off the stage.
You have to be thinking about these kinds of things in your matches. Make up your own signature style that works well for you.
Also work on that Dair. You had several chances to send Ness down to a well deserved death. A wasted opportunity can result in losing a match.
Good job on asking for criticism though, thats the fastest way you'll get better. Keep it up, keep practicing with Samus, and i'd say stay away from trying to find a secondary until you get pretty good with Samus, unless you think you have a character that you think you're better with instead of her, then i'd say switch boats and go for that one, you're at the point where its still easy and no drawbacks to changing your main. Just keep asking and looking at what the experts do on their videos to see if you can pick up any tricks. good luck. =D
Edit: I watched the second video too and noticed how committed you are you your attacks, which makes you easy to read. You also tend to repeat moves over and over if they don't work (when ness is on the ledge and you are fsmashing at least three times), and that only make you easy to punish. Also these two things make it so when the other guy makes a mistake, he rarely has to pay for it. Be versatile. Plan ahead but remember that you can't always stick to a combo or an attack.