I’ll try and help as best I can, but I’ll be honest, there are more advanced players here who could probably answer those questions much better. I’m trying as best I know.
If you are wondering about pressuring your opponent with Zero, Ama, Chun Li, or Tron, all these characters have good assists that can add pressure on and make it hard for your opponents to get the upper hand. However, depending on how you play any one of them as your point character can affect how your game works out as well. MvC 3 works different from Street Fighter in regards to rushdown and countering it. In SF, a rushdown character can often throw out normals with an unpredictable pattern to keep your opponent guessing/panicked and furious to escape that pressure. In MvC 3 that doesn’t work if your opponent knows how to advancing guard (pushback) so in that regard, rushdown changes. You’ll need more than normals to get in on your opponent, and that’s where good assists and mixups come into play.
In MvC3 since most good players youll come across will use pushback often to gain distance and reduce chip damage from beam attacks, you SF style rushdown has to be modified so you use your assists to force your opponent to block in a certain pattern, whilst your point character rushes in on the opponent locked down in blockstun. I don’t think advance guard can be used against assist attacks, and if your assist attack forces the opponent to block low, jumpin attacks will set you up for some hard to block/guess situations. Crossups also become important in a game where they are much harder to execute because crossup attacks will force your opponent to guess which way to block, and if they lose that coin flip, you open up a combo on them.
Zero’s got some really ambiguous mixups and crossups if you know where to start them and how to pin the opponent, and with x factor and shadow clone (or maybe not even need xfactor really) you can lock the opponent down for some nasty, hard to escape chip to death situations. Ama is also a good combo character, but has good assists, especially that ice beam assist, Chun Li’s Lightning Legs can be a good lockdown/juggle assist combined with OTGs, and Tron has lots of tools to hit, hurt, and make really long combos. She’s also hard to dodge because of her jumping heavy.
As for your execution, even in SF you should not be reckless and just throw out something on wakeup. Trying to wakeup with a special everytime isn’t a good idea, as even the simple shoryuken can be punish if thrown out at a bad time or baited.
Anyway, if you are impatient, nervous, or under pressure, it can mess with your inputs. If you go into training, I’ll bet you can throw out DPs and Hadokens easily like crazy. When in an actual match though, and under the pressure to win, you may find yourself getting nervous and messing up inputs. The best cure for that is to let go of your focus on winning, and more on learning, and staying calm. The more matches you play, the more you’ll learn, and winning will start to happen more, and become natural. You should be trying to learn about how an opponent will try to fight when he picks certain characters and how you could possibly counter it. If you can’t seem to counter it within the window of the match, take it to training, try to reproduce the situation as best as possible, and then find a counter and practice it.
In the end, the only way to get better at getting inputs right is to constantly be using them and practicing them. Play online, play against friends, do so in a light hearted, nothing at stake setting, and build up the pressure slowly. If you ever go to a tourney, you’ll be under lots of pressure, so you’ll have to learn eventually to stay focused and calm.
I know thats a long answer but I hope it helps.