I think so too.
Could be. From personal experience, nervous laughter is something my mother has always done. A LOT. Just as one picks up speech patterns growing up... so too the "add ons". Then I remember hearing that it was a very hard thing to "unlearn". This news did not please me. Still, being conscious of it did help to rein it in to a level that didn't bother me as much. Now, the most common time for me to laugh inappropriately is when I'm very hypomanic. There was one time at a meeting where there would be serious things, and I apologized in advance, saying if I had to leave the room, it was only because I couldn't stop laughing, even when I know it isn't funny at all. (They knew I was hypo, very, so they understood.)
Yes, here too. Do it all the time. Well, not sinister laugh, but just something random or goofy. I can amuse the hell out of myself, lol. Hey, why not, right? Sometimes the worse it gets, the more I joke. It's a defense mechanism for sure. I try to keep it in self-deprecation though and refrain from using it towards other's situations because it tends not to translate well. I have a dark sense of humor, and that doesn't bother me. A lot of time though, I think like what WNT2bNRML talks about -- if I didn't laugh, I'd cry. (Not that I don't do that too.) It just gets to such crazy levels sometimes, whatcha gonna do, you know?

A bunch of what you're describing though sounds like uncomfortable things. Like a coping mechanism that is bothering you, and like BlueInanna says, is something that can be worked with. You can still be goofy (yea!) and get to where the other is more comfortable.