Q1: What is the effect of "opponent cannot add memory except by tamer's effect?"

This effect prevents the opponent from adding memory to all effects except the tamer's effect. This effect affects the opponent player.

Q2: When "BT17-087 Marcus Damon Dai", which is treated as a Digimon, is rested while the opponent's "opponent cannot add memory except by tamer's effect" effect is activated, can't "BT17-087 Marcus Damon Dai" add memory by its All Turns effect?

No, it can add memory. Effects activated by cards treated as both Digimon and Tamer can add memory because they include both Digimon and Tamer effects.