There is nothing wrong with going to visit your friend or even boyfriend, but you should expect that they will probably ask a lot of questions. I went to the U.S. to visit my boyfriend for a 3 1/2 week visit once, but I was warned at the border that I couldn't just stay there permanently (and I had no intention of doing that).
The main thing is that they have to be sure you will go back home after your visit. If you can't convince them that you'll go back home, they might not let you visit, or they might restrict you to a short visit.
If you are coming just before the wedding, I would pay the PR application fee and have a copy of the receipt with you. I think you should get started on the paperwork now, so you'll be ready to send it once you're married. Having a printout of the draft application might be useful to show you are serious, but paying the $550 will be much more convincing. You just want to be able to show that you are going about it the legal way. Saying that you want to finish up the paperwork together is a good reason to come for a visit.
The thing is that you are not married or common law yet, so you can't actually be sponsored yet. There is another category called a "conjugal partnership" where you've become a committed couple living in a marriage-like relationship, but are prevented from living together. If you've been like that for more than a year, you could use that as an excuse until you get married, but it's probably not a valid way to apply in your case, so I probably wouldn't send in a conjugal application. By the way, your partner travelling for work doesn't necessarily mean you are no longer living together, but you do have to be together for 12 months for common law.
I think I would come for a short visit, get married, then go back home until the paperwork is done. If they let you stay, maybe you won't have to go back home right away.