I have highlighted some things for you to consider:

I call my boyfriend to cry to him about stresses at work and he gets irritated and tells me "I just got off the phone with my mothers emotions.. I can't deal with you too!"

Does anyone have any advice for me? I love my boyfriend so much. We have no issues in our relationship and he treats me like a queen! I just don't know if I can marry this monster in-law.

You do have an issue in your relationship. You are not first with your boyfriend and he feels free to say "I can't deal with you!" <--that is not treating you like a queen. The queen in this relationship is his overbearing mother.

You will not be marrying his mother; you will be marrying him. But that said, he will likely bring Mom into the relationship because she is still holding onto his backbone.

I suggest couples' counseling and Mom doesn't need to know about it. In fact, I would make this a deal-breaker. Your boyfriend has probably tried everything he can think of to have a 'normal' relationship with his mother. Time for professional help.

Stop trying to butter up his mother; treat her decently but stop her politely if she starts with the comments in your presence designed to cause trouble. If she'd made those inappropriate Valentine's day comments in front of me, I'd have told her with a smile on my face "But [boyfriend's name] is not your boyfriend" and if she is single, I'd also have said "I'd be happy to set you up with a profile on Match.com so you can have your own boyfriend."

The nasty gifts; same thing: politely say "Oh how lovely, unfortunately, I am allergic" It likely won't make her stop doing it, but everyone else in the room eventually will notice she mistreats you.

I married someone under his mother's thumb. We divorced; and one of the nastiest things he said to me (he thought) was "My mom said you never supported me." Not only was that not true in an emotional sense, but it literally wasn't true because at the time I was the only one working, so I was supporting him.

Keep in mind, too, that manipulative people will play people off each other. She might be going on and on about his twin to your boyfriend, and when she is with his twin, be going on and on about him.