[i][b]■What are great ways to keep players engaged both IC and OOC?[/b][/i]

The GM needs to be active and engaged themselves, lead by example. Make the OOC what you want it to be, talk to people, get to know them, have fun with the OOC section. It can be a great tool in keeping people engaged and just have fun outside of the game. It's also a great idea to use the OOC to make about IC events and situations and do planning and organization, instead of tying it all up in character. 

As for IC, I find you kind of have to find your pace and voice for it as a GM, and give players plenty of time for character interaction and quiet, fun moments along with action sequences. If things start to slow down, it's good to mix it up. Sometimes, a timeskip is necessary, and as a GM you need to make sure you have enough stuff lined up so each character has something meaningful to be a part of. In general, give players room to experiment and help make the story their own, but keep your vision and standards intact. Overall, it's a collaborate project, and if people feel like they're allowed to contribute to it and aren't just following a script, it enriches the game for everyone.

[i][b]■Should the character submission be the only factor in accepting players?[/b][/i]

I find sometimes it's best to go with your gut; I've found that if I got the impression an applicant had an attitude problem and ignored it, that would end up manifesting when they were in the roleplay and it created some unnecessary drama a few times. It is worth saying, however, sometimes you get the wrong impression and things can be a miscommunication over text, but following your guts is sometimes very helpful. 

As well, sometimes it's worth doing some background snooping on applicants and seeing how they get along in other roleplays and if there's any habits you should be concerned about. I know I personally have a few people "black listed" mentally because of shit like them joining and dropping roleplays like a chain smoker or combative attitudes. That said, if someone's enthusiastic and seems to be getting along with people, it's totally worth giving them a shot. Just make sure you don't compromise on the standards you set for your roleplay.

[i][b]■How do you deal with the players who have seemingly disappeared from the RP? 
[/b][/i]

Write their characters off after a period of time where that player is inactive. If a player can't be bothered to be a part of the game, then why drag the husk of their character along? Whether you kill that character off, have them GM puppetted into leaving the group, or even dropped without a word, it's up to you. Either way, if you spend time and effort on people who don't give a shit at the expense of those who are active and engaged. Those are your bed and butter players, they're the ones you should be trying to keep happy. That proverb about leading a horse to water rings true here.


[i][b][b]■In the same vein, how do you effectively prevent the above from hindering your other players?[/b][/b][/i]

Kinda covered it in the last part. Simply don't. If someone's holding something up and they aren't giving a valid reason for it, keep things moving along. If there's one person holding up several, then why wait for them to post after a reasonable amount of time (that's something you kind of have to judge on a case by case basis)? Just have the players continue on, and keep the plot moving. If the plot grinds to a halt, you're in for a bad time and people are going to start thinking the game is dying if it becomes a common enough issue. Keep shit moving, get off the beach, get to the choppah, etc.

[i][b]■What is the most graceful way to end your RP due to inactivity or (*gasp!*) your own loss of interest/time?[/b][/i]

Make an effort to keep it alive, express your concerns honestly with players, and if it's a lost cause, just make a post saying you're shutting down shop, and that you appreciate everyone's participation and that it was a good time while it lasted. Anything will do, as long as you're classy about it. I've had a couple times where I had a frustrating couple months but in the roleplay and in real life and after seeing it was pretty much dead, just tersely dropped a simple "No sense keeping this going." post and left it at that. Wasn't the most professional way to do it, but to err is human. Sometimes it's not anything you could fix that makes a game die, or it's multiple things that pile up. It's important not to blame any individual(s) for a game dying. 

But seriously, I was in the same boat as you years ago, and I realized that the only way I was going to see roleplays that I wanted to see was by doing it myself, and I haven't looked back. GMing is a lot of fun, can be very rewarding, and it honestly gives me a big sense of fulfillment when players react to things emotionally. It validates the hard work I put in, and it means that they're enjoying a shared experience. I think people also really appreciate having a platform where they can actually have time to develop their characters and their stories, since a lot of games fail for a lot of reasons, and I think a lot of the time is just lack of GM effort or focus.