Welcome to Flyertalk! You will need to research, read and read some more to answer some of your questions for yourself. This can be a rapidly changing game, so we only know what is today, not what will be tomorrow.

1. Targeted offers are mailed, emailed or otherwise personalized. Non-targeted offers are open to everyone.
2. Head over to the United Explorer thread in the credit card programs and spend some time reading it. There is a 50K offer that Chase will usually match if you send them a SM (secure message) asking them to match the better offer, within a certain time frame. Officially, you can't get the bonus again. Some people have been able to by waiting a year or more after closing the card. Plan to check the relevant thread occasionally for datapoints.
3. Chase Sapphire Preferred, Ink Plus, Ink Bold, United Explorer Business, Chase Freedom if you also have Sapphire Preferred. Also, for now at least, US miles can be used for flights on United flights, and the US card has no spending requirement.
4. Yes. Each card has its own rules. BofA Alaska may be the most churnable card right now, but again, rules change.
5. United for most people, with so many Chase cards that earn United miles or UR points that can be transferred to, among other things, UA miles.

Find what works for you, read and read more, subscribe to some blogs and read what they have to say about redeeming miles efficiently. Be ready to jump on a great deal when it shows up, but don't worry if you miss one. And travel on your terms, to the places you want to go.