You choose 1 of your Digimon for every 2 colors your Tamers have. The target Digimon aren't affected by your opponent's Digimon's effects until their turn ends.

This effect prevents Digimon from being affected by effects. For example, your Digmon won't suspend if it's chosen for a "suspend 1 of your opponent's Digimon" effect, and its DP won't be reduced by 3000 if it's chosen for a "1 of your opponent's Digimon gets -3000 DP."

Yes, it can be chosen. For example, a Digmon that isn't affected by effects can be chosen for a "suspend 1 of your opponent's Digimon" effect.

Yes, it can. It won't be affected by it, but it can be given an effect. However, if an effect such as <Security A.> is given to a Digimon that isn't affected by effects, the Digimon won't be considered to have that effect.

As soon as it gains the "isn't affected by [...] effects" effect, it will no longer be affected.

It will be affected by the effect as soon as it can be affected by effects.