Rulings
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 treated as a Digimon, it will gain inherited effects. In addition, it will be able to attack.
Q2: If a Tamer is 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 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 an effect of a Digimon or a Tamer?
When it activates an effect, it is considered both a Digimon effect and a Tamer effect.
Q4: 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.
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.
Data Sources
The Champion Ultimate Fighter! BT21-096 C [Main] For the turn, 1 of your [Marcus Damon]s is also treated as a 12000 DP Digimon, can't digivolve and gains <Rush>. Then, that Digimon may attack your opponent's Digimon. Your opponent's unsuspended Digimon can also be attacked with this effect. --- [Security] You may play 1 [Marcus Damon] from your hand or trash without paying the cost. Then, add this card to the hand.
The Champion Ultimate Fighter! BT21-096 C [Main] For the turn, 1 of your [Marcus Damon]s is also treated as a 12000 DP Digimon, can't digivolve and gains <Rush>. Then, that Digimon may attack your opponent's Digimon. Your opponent's unsuspended Digimon can also be attacked with this effect. --- [Security] You may play 1 [Marcus Damon] from your hand or trash without paying the cost. Then, add this card to the hand.
The Champion Ultimate Fighter! BT21-096 C [Main] For the turn, 1 of your [Marcus Damon]s is also treated as a 12000 DP Digimon, can't digivolve and gains <Rush>. Then, that Digimon may attack your opponent's Digimon. Your opponent's unsuspended Digimon can also be attacked with this effect. ---
日本一のケンカ番長！ BT21-096 C 【メイン】ターン終了まで、自分の「大門大」1体はデジモン• DP12000としても扱い、進化できず、≪速攻≫を得る。その後、そのデジモンで相手のデジモンにアタックできる。この効果では、アクティブ状態の相手のデジモンにもアタックできる。 --- 【セキュリティ】自分の手札/トラッシュから、「大門大」1枚をコストを支払わずに登場できる。その後、このカードを手札に加える。