Q1: Can I choose to not activate a When Digivolving effect on the placed Digimon card for this card's When Attacking effect?
No, you can't. This effect requires you to activate a When Digivolving effect if present on the placed Digimon card.
Q2: Can I use this card’s When Attacking effect to place 1 level 4 card with Pulsemon in its text that doesn't have a When Digivolving effect as a digivolution card?
Yes, you can.
Lv.3 Pal P-147 P Rookie | Vaccine | Beastkin [Digivolve] [Bibimon]: Cost 0 [When Attacking] [Once Per Turn] By placing 1 level 4 card with [Pulsemon] in its text from your hand as this Digimon's bottom digivolution card, activate 1 [When Digivolving] effect on the placed card as an effect of this Digimon. [Your Turn] While you have a Tamer, this Digimon gets +3000 DP. (Rule) Name: Also treated as having [Pulsemon].
Lv.3 Pal P-147 P Rookie | Vaccine | Beastkin [Digivolve] [Bibimon]: Cost 0 [When Attacking] [Once Per Turn] By placing 1 level 4 card with [Pulsemon] in its text from your hand as this Digimon's bottom digivolution card, activate 1 of the placed card's [When Digivolving] as an effect of this Digimon. [Your Turn] While you have a Tamer, this Digimon gets +3000 DP. (Rule) Name: Also treated as [Pulsemon].
Lv.3 パル P-147 P 成長期 | ワクチン種 | 獣人型 「ビビモン」:コスト0 【アタック時】[ターンに1回]自分の手札から、「パルスモン」の記述があるLv.4のカード1枚をこのデジモンの進化元の下に置くことで、置いたカードの【進化時】効果1つをこのデジモンの効果として発揮する。 【自分のターン】自分のテイマーがいる間、このデジモンをDP+3000。 〈ルール〉名称:「パルスモン」としても扱う。