If immigration needs some clarification on your application, they will call you in for an interview to pick up your card. If you were sleeping at the dorms, maybe you should have put the dorms as your address but it's still a grey zone that you can easily explain if you were spending more time at home than at the dorms. You have not done anything wrong and there is no reason why they would deny your PR card renewal unless they thought you were lying about your days spent in Canada.
Ideally you get your PR card in the mail, your friend would fed ex it to you and you would be able to return with it but if that doesn't happen, you can fly to Canada on your UK passport any time. Don't mention to the airline that you are a PR but when you arrive in Canada, you should say that you are and show whatever you have. Do you have your landing papers with you? That would definitely help. Otherwise the police report about the lost PR card as well as the letter that you applied should help convince immigration that you are really a PR.
If immigration wants to know if you are outside Canada, they can. They have access to flight manifests when they want to. If you are ready to return to Canada now, you should. At least make sure that you continue to meet the RO. The RO is not something you should only meet before you want to renew your PR card. You need to meet it all the time.
Right now, your days spent in Canada 5 years ago are moving outside the window of the past 5 years. They will have to be replaced with new days spent in Canada. Now, if you weren't in Canada 5 years ago, you would be ok because you would be replacing outside days with outside days so it's a non issue but if you are replacing Canadian days with outside days right now, you are getting closer to not meeting the RO again, day by day.
Immigration can examine your status any time you enter Canada so even if you met the RO when you applied for the card and even if your friend mails you the new card, you could still be examined for meeting the RO the next time you enter and if immigration thinks you don't meet it, they can report you so be aware of that and continue to calculate your days on a day by day basis to make sure you return in time and that you continue to meet the RO in the future.