Will subglandular implants affect my ability to participate in bench pressing/strengthening my pectoralis muscles? (photos)

You should be fine to have the implants placed beneath the muscle. Once you have fully healed from surgery and have the clearance from your surgeon you should be able to return gradually to your current activity level. There are a few benefits to under the muscle placement including lower incidence of capsular contracture, more natural look, and the benefit of the upper portion of the implant being disguised by the muscle.

Based on your photos, I would high recommend a dual plane pocket through the inframmary crease into the subpectoral pocket. I do not believe you would be happy with a purely subfascial augmentation especially considering you have some chest wall contour shape diferences.