Would doing this be legal?

Your answer is the answer to this question. If you woned the stainless steel retractable key system, would you want someone else to take yours...

Can I do this?

My advice is to do your own work and to not copy others, even if you change it a ittle bit.

How similar is to similar when registering a trademark?

Go online and look-up "likelihood of confusion." Read about that legal standard and it should help you determine an answer to your question.

Can I cut up pages in an old atlas to make jewelry?

You bought the atlas so you can cut it up. There should be no copyright infringement if you cut it at random and don't try to make the jewelry map...

You have to check in the state where you live.

Almost certainly copyright infringement if you use someone else's artwork on T-Shirts for sale.

Is it illegal to post photos of someone’s child without their consent if the photos were not taken in public?

Doubtful you could have someone arrested for copyright infringement.

Yes, you can sue them for not returning your deposit.

Can I get sued? ?

You can get sued for almost anything. So, yes you could get sued.

If the lanlord agreed not to cash the check, you could possibly have an arguement that he breached that agreement (oral contract). However, whether...