Does reapers toll
Break immune to physical with decrep?

yes

it can but breaking the immunity means not using other useful tools on your merc like pride. what i like to do if immunities need to be broken is simply use enchant or having a support char with high level amp damage. decrep casting area is low but amp damage is twice as effective and covers the entire screen. so instead of using reapers toll you can use a weapon with 5% chance of amp on striking. but being realistic all melee builds should use grief and you wont have that ability with grief so just use enchant.

Famine berserker axe/legendary mallet. Has magic damage + elemental damages

Reaper toll on a2 merc
Lawbringer on a5 merc
Atmas ammy

Just ignore phys inmunes as zealot, there's not a lot of them. Or put points in FoH/ Holy Bolt/ Blessed Hammer. Using azurewrath or other elements damage weapon on swap is possible, but CTA is nice, or Hoto for faster tele and spell casting, so not best having 2 melee weapons imo.