Winter Burial + Glacial Hammer + Melee Splash

How does it all stack?

Winter Burial will splash all cold damage, but only cold damage.

Melee splash will splash all damage, but with a penalty.

Using both will splash all damage, you will get the penalty, but you won't double up on splash damage as far as I know.

Thanks for helping. I got other questions, how does Hypothermia works when I'm attacking frozen enemies? Are frozen mobs also considered chilled or do I lose the 27% more damage?

Also, how to make the most out of that weapon?

Frostbreath is not really an endgame weapon

The most reliable method of Chill is Chilled Ground, aka Arctic Armour, Arctic Breath (usually on a trigger) or glove enchantment of Spite

Chill and Freeze are not mutually exclusive, so you can and do Freeze and Chill at the same time - the vast, vast majority of time, if an enemy is Frozen, they are also Chilled, with a few niche exceptions