Q1: If I have 2 Tamers in play, can this card's When Digivolving effect give up to 6 of my opponent's Digimon -5000 DP?
No. No matter how many Tamers you have in play, the effect gives -5000 DP to up to 3 of your opponent's Digimon.
Lv.4 Rapidmon BT8-039 SR Armor Form | Vaccine | Holy Warrior <Armor Purge> (When this Digimon would be deleted, you may trash the top card of this Digimon to prevent that deletion.)[When Digivolving] Suspend 1 of your opponent's Digimon for each Tamer you have in play. Then, up to 3 of your opponent's suspended Digimon get -5000 DP for the turn.
Lv.4 Rapidmon BT8-039 SR Armor Form | Vaccine | Holy Warrior Digivolve: 3 from [Terriermon].<Armor Purge> (When this Digimon would be deleted, you may trash the top card of this Digimon to prevent that deletion.)[When Digivolving] Suspend 1 of your opponent's Digimon for each Tamer you have in play. Then, up to 3 of your opponent's suspended Digimon get -5000 DP for the turn.
Lv.4 Rapidmon BT8-039 SR Armor Form | Vaccine | Holy Warrior [Digivolve] [Terriermon]: Cost 3 <Armor Purge> [When Digivolving] Suspend 1 of your opponent's Digimon for each Tamer you have in play. Then, up to 3 of your opponent's suspended Digimon get -5000 DP for the turn.
Lv.4 ラピッドモン BT8-039 SR アーマー体 | ワクチン種 | 聖騎士型 「テリアモン」から3≪アーマー解除≫(このデジモンが消滅するとき、このデジモンに重ねられているカードを上から1枚破棄することで、消滅しない)【進化時】自分のテイマー1体ごとに、相手のデジモン1体をレストさせる。その後、このターンの間、レスト状態の相手のデジモン3体までをDP-5000。