Woof.
OK, first, regarding the canon event. For the last X years (where "X" was the number of GenCons at which a BattleTech canon event was run ), the format for the canon event has always been "20 guys sit down at a table and play a WTFhuge game of BattleTech that either has an effect on the storyline or plays out a canon scenario". The gigantic downside to running a game this way is that, well, it's 20 people around a table at a convention. There's not enough space, fat people crowd everyone else away from the table so you can't see it, people take FOREVER to make their moves, and over the course of a whole day we get 12-15 turns done. Tops. Individual people aren't actually getting to play that much "BattleTech" during that time; they're controlling 1-3 mechs (so, 40-60 units on the board) over 12-15 turns. But it takes 12 hours. Thus, it was decided this year to try a new format.
The new format (known as Vimy Ridge Rules) essentially splits the single huge game into a dozen or so smaller games that are run simultaneously. Each individual table has 2-4 players, with each player controlling 2-6 Mechs. Each Team has a single commander who coordinates the tables ("you on Table 4 need to reinforce Table 7 to prevent the OPFOR from breaking through to Table 8 where our supply depot is") and any strategic assets the forces may have. The primary strategic force will be areospace support - simultaneously with all the tables being played, there's an AreoTech game going on on yet another table, which is mapped over the BattleTech tables. Units from the AreoTech game may make ground attacks as directed by the Team Commander on a given table...assuming they're able to do so whilst being opposed by OPFOR ASFs. What this means for the individual player is that they will get a LOT more actual "playtime" for their dollar. Based on testing at other cons, we're expecting to run a complete Turn every 20-25 minutes. Which is double what we were getting before. Plus you're actually at your table for the full time, rather than sitting for 90% of the day waiting for your turn to come up (ie, not actually playing the game). It's a LOT more complicated to run, and we're expecting some logistical delays as a matter of course, so be warned.
And no, you don't have to be there the ENTIRE time. However, I'd recommend devoting an entire day to it; 3, 4-hour blocks. There's a sense of continuity you don't get if you do 1, 4-hour block per day that makes the canon event unique.
2) Scheduling.
Origins is a good warm-up for GenCon, because they're the same general format. GenCon is just an order of magnitude larger. So, what I recommend for the 1st time con attendee is to schedule 2-3 things you KNOW you want to do. Go do the canon event for a full day (I recommend Friday). Leave 4-8 hours to wander the exhibition hall (Thursday and Sunday). Take at least a few blocks of time and leave it open (Saturday, late Thursday) to go around with generic tickets and drop in to stuff that looks "neat". Basically, schedule in a little bit of everything, and don't lock in your schedule too tightly so EVERYTHING is pre-planned. Food is grabbed on the fly - while there's technically no time listed between an event during Block 1 (10-2) and Block 2 (2-6), in reality, Blocks end 15 minutes early. This allows you time to grab food and/or bathroom breaks, and gives the GMs time to catch up on their paperwork (I leave out sanity, because that's what the booze parties in the hotel rooms are for - be nice to your GMs in the morning slots; hangovers are killer). Sometimes, you can duck out of a game for a few minutes to grab food or a bathroom stop; just check with the GM first - do NOT just walk off. Evening tables will every often coordinate for food runs out to the food court so only 1 person has to go.
3) Leviathans
No. No tourney. As the person who wrote the tourney, I will remain silent on what I think about this. You can probably go straight to KotS, honestly. Just be aware that we are finally (FINALLY!) supposed to be getting actual rulebooks for the GMs to look through. This means some rules may change from what you've experienced (see also: bracketing fire, I think we're on iteration 12), and mistakes from the rulebook will be prevalent as our GMs learn the "published" game and its differences from our "playtest" game.
4) 1st-time attendee
Honestly, I can't think of anything. Everyone will have different priorities. Otaku Hambeast the 400# Cosplaying Anime Catgirl will want to ensure she hits different things during the con than, say, Dan would. Basically, the single biggest thing is to ensure that you don't over-schedule yourself. Leave time to wander the exhibition hall and get your breath back.