See Damage#Offensive for mechanics.
The glyph from Cawwot (spell) counts as being placed by the caster, so it is based on the caster's statistics and not the summon's.
The effectiveness of a summon's healing spells are increased by 1% for each of the character's levels. Summons' heals are not affected by the intelligence of the summoner.
Note: The Barrel summoned by Drunkenness only heals while being carried.
See Healing weapons for a list.