Alot of the answer will depend on the contract. Who gwts editorial control? If you do, tell them the work is done. If they do, you will probably have to go along unless you can show they have breached the contract. It is time to hire a lawyer.
It may be copyright infringement to copy. An interesting question is who is the owner of the copyright? If the t-shirts go huge you will probably find out. Otherwise, the owner of the copyright will probably want to keep quiet, unless he or shee had permission. If you know who it is just get a license.
The fact that you all sigbed the lease does not necessarily mean she is breaching a contract with you. You probably want to get any attorney to review the lease. Bit may give you veto power over the new tenant, but it may not. Run ads on your own for a roommate so you have some options.
You should get a copy of the police report and take it into the bank. That should unfreeze your account. However, you will probably have to reimburse the bank for the fraudulent check. If that does not work, go see an attorney
You can you the trademarks of others if they are necessary to state a fact, I.e. there was a robbery at McDonald's. You cannot use the trademarks in any way which suggests an affiliation with the owner. Thus, yoi are fine as long as you are not creating a likelihood of confusion. Please understand that there are some companies that are hypersensitive about there marks and will threaten people who are using the marks properly - you know "the big game".
Simple resolution. Stop stealing other people's content and you can avoid demands to pay them money. The porn cases usually go for more because most people do want their friends and family to know their viewing habits.
This is pretty common. If you ise something without paying, yoi will eventually get caught. The amount they are asking is high and yoi can probably talk them down to $1-2,000. They do not want the expense of the suit and judges will often reign in a copyright holder who they think is making excessive demands. Go see a good copyright attorney ASAP.
I think Bruce is right. The reason that the provision is in your agreement is to address this very type of situation. They do not want you using "their" projects and holding them out as your own. The fact that they fired you (as opposed to a lay-off) suggests that they are probably unlikely to agree. I have not seen the exact language, but you might be able to contact former customers and ask if you can come photograph your place. Then do not call it "your" work, just indicate that they...
Legal answer - it depends on the exact terms of any oral agreement rewho owns the artwork. Most likely you do.
Business answer - it will cost your business reputation many times that $75 if you sell the image to someone who is going to knock of their logo.