Indeed you are free to use them as long as you meet the rank requirements. Some jutsu have notes on though saying "Can only be used by X", just beware of that.
Yes that list is basically what needs to be bought, some may be a little outdated. You can go to the rules forum, special bios and abilities, click on each ability there and at the bottom it should tell you the cost if you need to buy them too.
To buy custom jutsu, you speak to other players and come to a deal, based on the value you both agree on. I'd advise you don't buy any just yet to be honest, save your kumi and i'll help you when it comes to investing.
Cannon wise, you buy the cannon ability then learn all the jutsu, don't have to buy the jutsu, just the ability, then you get trained in said ability.
If you submit it as a single mission it will only count as 1 D rank mission, which you will need to pass to count towards ranking up.
Once your bio is approved, you can't edit that thread. If you want to update it, you post a new thread in the bio thread and at the bottom, you post a link saying "Updating this bio"
Yeah you can keep editing your bio in there, but to update it, you have to submit it for checking again.
So basically everytime I learn new ability or rank up I need to make a new thread updating my bio before it actually can be used?