Q1: What does this card's "can't reduce digivolution costs" effect do, exactly?

It negates effects that read, "reduce the memory cost of the digivolution by X," like Digisorption and BT3-103 Hidden Potential Discovered!. Even if your opponent activates effects like these, it won't reduce the memory cost of their digivolutions. They have to pay the digivolution cost on the card in order to digivolve.

Q2: My opponent has this Digimon in play. If I activate Digisorption on a card in my hand, will I be able to reduce digivolution costs with it?

No, you can't.

Q3: My opponent has this Digimon in play. Can I use effects like BT2-111 Beelzemon's "digivolve into this card in your hand for a memory cost of X, ignoring this card's digivolution requirements" effect?

Yes. BT2-111 Beelzemon's effect doesn't reduce digivolution costs--it allows you to ignore digivolution requirements and digivolve for a specific memory cost, so you can still activate it.