I just fought a Rey with my DR on TW, deathmark does not reduce Max health, it does that quanitity of dmg
For example, if an enemy has 10,000 HP, hitting him will deal 5,000 HP plus the dmg from the attack itself, but his Max health will still be 10,000
And no, it does not "surpass" dmg immunity because it is not reducing max health, however, AFAIK, a character Max health CAN be reduced even if he has dmg immunity, for example GAS hitting the toon without protection
TL;DR
Deathmark does not reduce max health, it does damage based on it
Dmg does not surpass dmg immunity, but max health reduction does