Q1: What does an "is also treated as a Digimon with X000 DP" effect do, exactly?

It causes that card to also be treated as a Digimon with X000 DP. For example, if a Tamer is also treated as a Digimon, it can attack like a standard Digimon, and it will gain inherited effects from cards stacked under it. However, even if a Tamer is played and then treated as a Digimon in the same turn, it can't attack during that turn.

Q2: If a Tamer is also treated as a Digimon using an "is also treated as a Digimon with X000 DP" effect, is it no longer considered a Tamer?

No, it is treated as both a Digimon and a Tamer.

Q3: If a Tamer is also treated as a Digimon using an "is also treated as a Digimon with X000 DP" effect and it activates an effect, is it treated as both a Digimon effect and a Tamer effect?

Yes, it's treated as an effect of both. Because the card is treated as both a Digimon and a Tamer, the activated effect is treated as both a Digimon effect and a Tamer effect.

Q4: If a Tamer is also treated as a Digimon using an "is also treated as a Digimon with X000 DP" effect, then another effect causes its DP to become 0, is it deleted upon a rule check?

Yes, it's deleted upon the rule check timing.

Q5: If this card's Main effect is used to treat Marcus Damon as a Digimon, can I use the part of the effect after "then" to attack using that Marcus Damon?

Yes, it can attack.

Q6: If this card's Main effect is used to treat Marcus Damon as a Digimon, can I choose to not attack using the part of the effect after "then"?

Yes, you can.

Q7: If a Tamer is also treated as a Digimon using an "is also treated as a Digimon with X000 DP" effect, and later it gains another "is also treated as a Digimon with X000 DP" effect, what happens?

If an effect has triggered and the card is affected by a "is also treated as a Digimon with X000 DP" effect, the play cost, level, and DP are overwritten by the newer effect. However, if an effect has already activated and the card is affected by a "is also treated as a Digimon with X000 DP" effect, the already activated effect will be overwritten. Any other effects such as Rush that are later gained will be added.

Q8: Can I gain memory using an effect on a Tamer that's treated as a Digimon with X000 DP due to an effect while my opponent has "your opponent can't gain memory other than by Tamer effects" activated?

Yes, you can. When an effect is activated on a card that's treated as both a Digimon and a Tamer, the effect is treated as both a Digimon effect and a Tamer effect, therefore you can gain memory.

Q9: If an opponent's card has "isn't affected by Digimon effects" and it's chosen for an "is also treated as a Digimon with X000 DP" Tamer effect that treats it as a Digimon, will the chosen opponent Digimon be affected by effects?

No, it won't be affected by effects. When an effect is activated on a card that's treated as both a Digimon and a Tamer, the effect is treated as both a Digimon effect and a Tamer effect, therefore the chosen opponent Digimon won't be affected by effects.