Can [BT1-105 Blast Fire] to set the DP of Eldradimon?
Yes, Eldradimon's effect prevents the reduction of DP. But it does not prevent its original DP from being set to another number.
My opponent uses [BT3-101 Bifrost] and targets this card. Eldradimon is then hit by a <De-Digivolve>. Does the new Digimon have the decreased DP?
Yes, the new Digimon has -3000 DP.
My opponent uses [BT3-101 Bifrost] and targets this card. I digivolve Eldradimon into a new Digimon. Does the new Digimon have the decreased DP?
Yes, the new Digimon has -3000 DP.