No. Guinea pigs breed very quickly, there would not be time to stop them, and you just be inviting unwanted pregnancy this way. Aside from that risk guinea pigs should not do "play dates" as they are stressful on the guinea pigs. Guinea pigs need to establish a hiarchay and stay within that to feel safe and comfortable with each other. When they are brought together and then parted it is unnecessarily stressful on them.
No, definitely not. It only takes seconds for a male to impregnate a female, like three seconds. You could yawn, sneeze, answer the phone, look away for a minute, whatever, and completely miss the whole scene. If you want them to have companionship, either get neutered or adopt one more of each sex to go with the two you have.
You could have your boar neutered, or your sow spayed, so they can live together permanently if you want them to interact. Or, just get them new cagemates. But definitely no 'play dates' because you'll end up with lots of new cagemates if you know what I mean!
No he shouldn't be in pain because he should have meds for that. The reason why I say he is because spaying a female guinea pig is quite complicated, I recommend neutering over spaying. Just make sure you find a really good exotic vet.
Neutering/spaying is a surgery, so it does hurt them for a short amount of time after they wake up from the anistesisia, but with proper pain meds and care, they really do recover rather quickly and can happily live with your other pig just like they always would but without the risk of pregnancy.