Neutrons are not made of an electron, antineutrino and a proton. However, you can get those out of a neutron through beta decay. Neutrons are made of an up quark and 2 down quarks. The quarks do have electrical charge (up = +2/3, down = -1/3. The third is purely due to the long history of calling electrons -1 and protons +1). Essentially, what happens is a down quark converts to an up quark by emitting an electron and neutrino. You could also do the opposite, known as inverse beta decay (this is pretty difficult, if I recall, due to the rare neutrino interaction. But I'm pretty sure this is how they detect neutrinos).
If you are asking what the force is that causes AM/M annihilation, there is none. They are basically waves of opposite amplitudes. So the electron is the crest and the positron the trough. when they meet, they cancel each other out. However, these are not virtual particles. They both have energy. That energy has to go somewhere. E=mc^2, so something out of the quantum foam gains positive energy and this is a gamma ray.
As for antimatter experiments, the deal is that the LHC is making full anti-atoms of hydrogen (anti-proton orbited by a positron) and trapping them for long periods of time (I think they had some for 15 minutes). Since full anti-atoms do not naturally exist in nature, the idea is to look for differences in the way anti-matter behaves as opposed to matter. We know there has to be some difference due to the fact that all around us we have matter, and pretty much no anti-matter. The big bang should have made equal amounts of matter and anti-matter, so some crucial difference must have allowed only matter to survive. So, anti-matter isn't really all that special. It's rare and has the opposite electrical charge, that's all. So, anti-matter can't have negative mass. That would be a different class of anti-charge (thinking of mass as a charge) that I believe is known as exotic matter. So far, none has been found. Also, positrons are fairly frequent in the natural world, so it's safe to assume that would have been measured fairly quickly after their discovery. Random interesting fact: Positronium = positron orbited by an electron. I think they might have just made anti-helium or anti-deuterium somewhere too, so I think that covers the positron interaction with a neutron.
Personally, I think that the classification of which particles are group together and called matter and which are anti-matter might be messed up. I kinda think our world is a mix of matter and antimatter, the fact is something on average gained traction that caused the current assortment of particles, however some are matter, and some are anti-matter. Classifying 'the stuff we dont see' as anti-matter is kinda misguided I think, but what do I know?
You're probably about to jump me right now. "Ah ha! But you said virtual photons pop into existence, and annihilate each other! However, photons do not have electric charge!" That's right. Furthermore, is there an anti-photon? No, the photon is it's own antiparticle. It's a very fine line as to what is a virtual particle and what is a "real" particle. The only real way to separate them is to say that real particles is what our instruments pick up. Virtual particles are undetectable. Photons can interact with each other via virtual-electrons. You can probably tell by this point that any calculation about anything could involve an infinite number of these virtual particles, and that's true. It's a problem, but I digress: the best way of thinking about it without going through an infinite hall of mirrors is to just assume that a virtual photon and a virtual anti-photon (just use the crest/trough idea) pop into existence, and pop back out. You can wrap your head around that. Otherwise you got a chicken and the egg scenario. The way around that is renormalization; thats french for cheating, but accepted because no one knows how to get to the answer.
Also, some people think that all photons are virtual. I mean, if you think about it, you can't actually measure a photon directly. In regards to the negative mass, it's because the other particle escapes that it becomes real. They have a built-in self destruct. If one finds a way around that, it has to take energy from somewhere, and since they're entangled, the one falling in transfers energy out. It does seem like voodoo, but that's the best way I can explain it. Interestingly, this also pulls information out of the black hole. So, during evaporation, you would come out of the black hole bit by bit. When it evaporates completely, its not like everything it sucked in is suddenly released. The black hole is slowly reduced in size and mass until theres not enough mass to make it a black hole anymore. A large amount would theoretically be released then.