Multiple PCC is a standard term that shows on everyone's checklist. In your case, you only need to submit UK PCC. For your stay in Canada, you need not submit PCC as part of eAPR submission. While your application is being processed, in case CIC needs RCMP Clearance they will ask for it then
The instructions for passport upload does not say anything about old passport(s). It only asks for personal details pages, pages with visas, stamps, entries, work permit, etc to be submitted. So logically, this would be for the valid passport, the details of which you entered under Passport Section. However, it is a personal choice whether you want to submit pages of your old passport. For some people, if they include old passport pages (2-3 passports), it will exceed the file size limit of 4MB. So would leave it to you to decide.
Note: I have never come across a single case where application was rejected due to non-submssion of old passport pages.