Q1: Can I use this card's When Digivolving effect to give "Start of Your Main Phase This Digimon attacks" to a Digimon that isn't affected by effects?
You can give that effect to such Digimon, but Digimon that aren't affected by effects won't be affected by it, therefore the gained effect won't trigger.
Q2: If I use this card's When Digivolving effect to give "Start of Your Main Phase This Digimon attacks" to a Digimon, can I choose to not attack for the part of the effect after "then"?
Yes, you can.
Lv.6 Gaiomon P-183 P Mega | Virus | Dragonkin/X Antibody <Reboot> <Blocker> [When Digivolving] Until your opponent's turn ends, 1 of your opponent's Digimon gains "[Start of Your Main Phase] This Digimon attacks." Then, this Digimon may attack. [All Turns] [Once Per Turn] When attack targets change, trash your opponent's top security card. (Rule) Name: Also treated as having [Greymon].
Lv.6 Gaiomon P-183 P Mega | Virus | Dragonkin/X Antibody <Reboot> <Blocker> [When Digivolving] Until your opponent's turn ends, 1 of your opponent's Digimon gains "[Start of Your Main Phase] This Digimon attacks." Then, this Digimon may attack. [All Turns] [Once Per Turn] When attack targets change, trash your opponent's top security card. (Rule) Name: Also treated as having [Greymon].
Lv.6 Gaiomon P-183 P Mega | Virus | Dragonkin/X Antibody <Reboot> <Blocker> [When Digivolving] Until your opponent's turn ends, 1 of your opponent's Digimon gains "[Start of Your Main Phase] This Digimon attacks." Then, this Digimon may attack. [All Turns] {{OncePerTurn}} When attack targets change, trash your opponent's top security card.