There are some "rules" that will make a download not start, like if there is a 2 file download limit, and you're trying to download a third file, for instance. Forced start (as well as forced seed) will ignore all those rules and just start the download (or seeding).
Yep, there is, just go to the configuration menu, Queue sub-menu, and in the "Max simultaneous downloads" entry just put whatever value you like (0 for unlimited simultaneous downloads).