1- Nonsuper invincible start-up moves exist but they are made so that they can be hit out of. If there's a super armor (MBAC term for invincible frame) frame, the opponent can repeatedly hit you without delay (ex. repeatedly using "a") and hit you before your move after the super armor frame is over even if it was to continue into an attack. Nobody intentionally uses the super armor frames to counter attacks as the game's too fast paced.
2- The most important thing is blocking. One hit can lead into combos that take 1/4~1/3 of your life. Shield is used mostly (and only) when the character has a good skill to follow up (ex. Vampire Sion's 623C that leads into a combo), but there's too much risk involved. Many times, shielding projectiles and summons can get you in a worse position. It's not like SF 3rd strike where more you can tech the better it is. If you watch top players, many of them don't ever shield at all (though I saw Arlieth do it many times). Shield bunkers... can be hit out or avoided... I don't know of any normal or top players that use it effectively.
4- "Invalid" means that it isn't a combo. That is, if the opponent does a recovery move, that move won't hit the opponent. A trick to making them not work... I'm guessing you mean not to make it count as invalid? If so, you must first do a combo string that is guaranteed to combo, and you must get the timing right. No tricks really, only practice.