I've had Myotonia since i was a toddler. I get bad tongue & facial cramps too but i have also had my tongue pierced for 5 or 6 years. The cramps in my tongue have never been stimulated by my tongue bar to my knowledge. When i do get tongue cramps my tongue bar doesn't get in the way really either. So I wouldn't fear the Myotonia as a reason not to get it pierced but your MC might be different and/or worse than mine so its all what you feel comfortable with & what you feel is right for you. You can always take it out if it doesn't work for you.
If you do go ahead, make sure you choose a respectable piercer with a good reputation & clean environment etc but im sure you know all that. When mine was pierced i think they just used some kind of numbing spray or gel which didn't cause me any problems.. & it never hurts as bad as you imagine either!
As for your teeth, having your tongue pierced is a risk you take. I've taken a bite out of mine a few times but my teeth are still ok. At first its hard to leave it alone like an ulcer your tongue wont stop playing with it! But now its like a part of my mouth. I have more trouble with repeatedly biting my cheeks than i do my tongue bar!