1. No, but please double check here -
https://www.gov.uk/check-uk-visa. I did, and it says
Exemptions
You don’t need a visa if you have one of the following:
a visa for Canada, New Zealand, Australia or the USA (this can be used for travel to any country)
2. I'd recommend a 4 hour minimum. Heathrow can take ages sometimes and you want to build in some time to account for any flight delays. Anywhere between 4-6 hours should be fine.
3. No, your bags will be checked in at the origin airport (Pakistan) all the way to your destination airport (Canada)