Well, first off you dont want to just run in and grab them. its all about mind games. lets say you run in and bat them (still far away enough so they cant grab you, i.e. minding your spacing), and then run away and run back and grab them. The idea is you dont want them to guess your going to grab. You want them to think your going to do an attack that they have to shield. Once youve got tehm thinking that, they aren't oging to grab out of the shield (or esle they'd take damage). You get what im saying? So practice Crouch Cancelling the bat (i.e. running and then tapping down before pressing forward on the cstick), as this will alllow you to bat out of a dash. Also practice running and then wave dashing back, and hten forward again.
Your question is basically "how can i grab them without being predictable so they grab me first", and the answer is to either mix up long range attacks that force them to shield so they think your going to do one of those attacks when your actually going for the grab, OR its to confuse them into the timing of when your going to grab. So, you might run up to them, and right before your close enough to them so that you can grab or they can grab, you wave dash back (at which point they will shield grab since they would have thought you were going for the grab) and then dash forward. Now your next to them and they are probably facing the lag of their grab. You can
a. grab them
b. attack them
i suggest a.
as for missile spammeage, my greatest suggestion is simply dodging them. The key to dodging samus's missiles is to not be intimidated by them. Just dodge them. Shield and wave dash forward, wave dash forward while spot dodging, short hop and air dodge past them, full jump and air dodge when you need to (although the disadvantage in this is you can be an easier target in the air when you full jump).
btw, this is Livevil200. This will be my new name.