Q1: Can I also use this card's inherited effect to switch the attack target to an unsuspended Eosmon?
Yes, you can.
Q2: If both players don't play a Tamer for this card's On Play When Digivolving effect, do I still perform De-Digivolve 1?
Yes, you do. You perform the part of the effect after "then" regardless of whether or not a Tamer was played.
Lv.5 Eosmon BT17-075 U Ultimate | Unknown | Unknown [Digivolve] Lv.4 [Eosmon]: Cost 3 [On Play] [When Digivolving] Your opponent may play 1 Tamer card from their hand without paying the cost. If this effect didn't play, you may play 1 white Tamer card with a play cost of 4 or less from your hand without paying the cost. Then, <De-Digivolve 1> 1 of your opponent's Digimon for every 2 Tamers. --- Inherited: [Opponent's Turn] [Once Per Turn] When an opponent's Digimon attacks, you may switch the target of attack to 1 of your [Eosmon].
Lv.5 Eosmon BT17-075 U Ultimate | Unknown | Unknown [Digivolve] Lv.4 [Eosmon]: Cost 3 [On Play] [When Digivolving] Your opponent may play 1 Tamer card from their hand without paying the cost. If this effect didn't play, you may play 1 white Tamer card with a play cost of 4 or less from your hand without paying the cost. Then, <De-Digivolve 1> 1 of your opponent's Digimon for every 2 Tamers. --- Inherited: [Opponent's Turn] [Once Per Turn] When an opponent's Digimon attacks, you may switch the target of attack to 1 of your [Eosmon].
Lv.5 Eosmon BT17-075 U Ultimate | Unknown | Unknown [Digivolve] Lv.4 [Eosmon]: Cost 3 [On Play] [When Digivolving] Your opponent may play 1 Tamer card from their hand without paying the cost. If they don't, you may play 1 white Tamer card with a play cost of 4 or less from your hand without paying the cost. Then, <De-Digivolve 1> 1 of your opponent's Digimon for every 2 Tamers. --- Inherited: [Opponent's Turn] [Once Per Turn] When an opponent's Digimon attacks, you may switch the attack target to 1 of your [Eosmon].
Lv.5 エオスモン BT17-075 U 完全体 | 不明 | 不明 Lv.4の「エオスモン」:コスト3 【登場時】【進化時】相手は相手の手札から、テイマーカード1枚をコストを支払わずに登場できる。この効果で登場しなかったなら、自分の手札から、白の登場コスト4以下のテイマーカード1枚をコストを支払わずに登場できる。その後、相手のデジモン1体をテイマー2体ごとに、≪退化1≫。 --- Inherited: 【相手のターン】[ターンに1回]相手のデジモンがアタックしたとき、アタックの対象を自分の「エオスモン」1体に変更できる。