Does activating a phone at e.g. BestBuy Mobile include carrier promos?

For new customers yes, the promo is the same in a store like best buy, and best buy usually throws in some bonuses like a gift card of some sort.

For renewing customers, sometimes they will throw in a gift card as well whereas a corporate store wouldn't. So it's always better to go though bestbuy/superstore/walmart for new customer/upgrade deals.

Pro Tip: Before renewing, check with your favorite retailer (I like loblaws cause PC points) what the special is for the phone you want. Once confirmed, call up retentions and negotiate a deal with the rep(bill credit, lower monthly fee etc etc), and once confirmed, tell them you will go to the store to pick up the phone. Return to best buy and take advantage of the renewing customer deal from them, and then call back customer service to say you got the phone, and to apply what was discussed (hopefully they left notes or gave you a reference number)

Also check to see if you may have any kind of corporate employee purchase plan deals available, depending where you work.