No, it isn't. It will only be treated as a Digimon if an effect treats it as a Digimon.

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.

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

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.

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

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.

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.

Yes, you can. Even if you pay a cost and the memory moves to 1 or more on the opponent's side during the main phase, first any effects that triggered during that timing must resolve, then the end of the turn timing occurs. In this case, once all of the attacks from [RizeGreymon]'s [On Play] [When Digivolving] effect have resolved, then the end of the turn timing will occur. Because the attack has already resolved and isn't currently occurring, you can activate this card's [End of Your Turn] effect and have 1 of your Digimon attack.

No, an attack using the 2nd effect isn't possible. A new attack declaration can't be made during an attack. In this case, the [End of Your Turn] effects on both copies of this card trigger simultaneously at the end of your turn, and you use the [End of Your Turn] effect on the 1st copy of this card to attack. The 2nd [End of Your Turn] effect can be activated before the counter timing, but because you can't declare another attack during an attack, you won't be able to attack using the 2nd effect, even if you activate it.

No, you can't. If your turn ends while you have a yellow Digimon with [Agumon] or [Greymon] in its name in the battle area, you must choose 1 of your [Marcus Damon]s, and it will gain the "is also treated as a 6000 DP Digimon, gains <Rush>, and can't digivolve" effect. Then, you choose whether or not 1 of your Digimon will attack using the part of the effect after "then." At such times, [Once Per Turn] is considered to be used, even if you don't have 1 of your Digimon attack using the part of the effect after "then."