1. How do you race each other in online races?
More specifically, how can you actually race each other (where you can hit them) and not a "ghost" of your opponents?
There are two ways you can do it:
1) Race in Landrush and Rallycross. These will always involve bashing into one another.
2) Go into Jam Session and either find or make a game that uses staggered starts.
I cannot, however, recommend either option. Players have the disturbing habit of bashing into anything that moves, particularly in Rallycross and Landrush. If you're not on the front row of the grid, you're pretty much screwed. And it's really not very fun.
2. Are there any unlockable game modes?
I can't seem to find Outbreak.
Outbreak can only be played online. Again, there are two ways of doing it:
1) Go into Pro Tour and check out Party Modes. These will cycle you through Transporter, Invasion, Outbreak and - if you're playing in teams - Cat 'n' Mouse.
2) Go into Jam Session and select Custom Game. Scroll through the discipline options until you find the one you want. Leave everything else set to "any", and the game will search for existing games that match the criteria you set.
I'd recommend you do the first option. Constantly playing one of the party games gets boring quickly. Especially if one person knows the Battersea Compound very well.
3. Are there any unlockable cars?
Not really. There are only liveries that can be unlocked. However, if you play the Dirt Tour in such a way that you always take the most-recently-available team to get the highest reputation bonuses, you may not encounter some cars that have always been available until very late in the game. For instance, Fidanza run a Peugeot 405 in the Classic Trailbalzer class. However, by the time you get to races that run these cars, you will have graduated from the Fidanza cars. I didn't even know there was a 405 in the game until I unlocked the DC Shoes Race Works team at lvl. 27 simply because I'd bypassed the other teams that ran the 405 until the final races.
So pretty much every car is availabe at the start of the game. You just need to know where to look. Although there are some special liveries (bronze RS200, silver Touraeg 2, gold PM580 and platinum Fiesta) that can only be unlocked online. Everything else will be DLC.
4. How long can your YouTube videos be?
It seems that I can't record videos longer than 1 minute.
The maximum length of videos was deliberately capped to keep down the time it would take to upload the video. If you want to make longer videos, you'll need to use a third-party recording program.