Q1: Can I use this card's Main effect to pay 1 cost even when I don't have any Legend-Arms trait or level 4 Digimon?
No, you can't. The activation conditions include paying 1 cost and placing this card as 1 of your Legend-Arms trait or 1 of your level 4 Digimon's bottom digivolution card, therefore the effect can't be activated just by paying 1 cost.
Lv.4 TiaLudomon EX6-040 C Champion | Data | Armor/Legend-Arms [Digivolve] Lv.3 w/[Legend-Arms] trait: Cost 2 [Hand] [Main] By paying 1 cost and placing this card as 1 of your [Legend-Arms] trait or 1 of your level 4 Digimon's bottom digivolution card, that Digimon gets +2000 DP until the end of your opponent's turn. [Your Turn] [Once Per Turn] When an effect adds to this Digimon's digivolution cards, it gains <Blocker> and <Reboot> until the end of your opponent's turn. --- Inherited: [Opponent's Turn] This Digimon gets +2000 DP.
Lv.4 TiaLudomon EX6-040 C Champion | Data | Armor/Legend-Arms [Digivolve] Lv.3 w/[Legend-Arms] trait: Cost 2 [Hand] [Main] By paying 1 cost and placing this card as 1 of your [Legend-Arms] trait or 1 of your level 4 Digimon's bottom digivolution card, that Digimon gets +2000 DP until the end of your opponent's turn. [Your Turn] [Once Per Turn] When an effect adds to this Digimon's digivolution cards, it gains <Blocker> and <Reboot> until the end of your opponent's turn. --- Inherited: [Opponent's Turn] This Digimon gets +2000 DP.
Lv.4 TiaLudomon EX6-040 C Champion | Data | Armor/Legend-Arms [Digivolve] Lv.3 w/[Legend-Arms] trait: Cost 2 [Hand] [Main] By paying 1 cost and placing this card as the bottom digivolution card of 1 of your Digimon that's level 4 or has the [Legend-Arms] trait, that Digimon gets +2000 DP until the end of your opponent's turn. [Your Turn] [Once Per Turn] When an effect places a digivolution card under this Digimon, this Digimon gains <Blocker> and <Reboot> until the end of your opponent's turn. --- Inherited: [Opponent's Turn] This Digimon gets +2000 DP.
Lv.4 ティアルドモン EX6-040 C 成熟期 | データ種 | 防具型/Legend-Arms 特徴に「Legend-Arms」を持つLv.3:コスト2 [手札]【メイン】1コスト支払い、このカードを特徴に「Legend-Arms」を持つかLv.4の自分のデジモン1体の進化元の下に置くことで、相手のターン終了まで、そのデジモンをDP+2000。 【自分のターン】[ターンに1回]効果でこのデジモンの進化元が増えたとき、相手のターン終了まで、このデジモンは≪ブロッカー≫と≪再起動≫を得る。 --- Inherited: 【相手のターン】このデジモンをDP+2000。