Is it better to have a robe that gives + mana or + int?

Magical gossamer robe, Defense: 0 to 8, +12 Intelligence

a robe with + int is much better

int gives you more dmg on your charm and very little mana

mana alone gives only mana ^^

unless you're very low lvl and playing caster already, then a mana robe can be usefull

Lv26, 97 int with robe 2058 mana, I usually try to hea,help the team last longer,l but I have a trash heal.
Rare focused heal VI, Heals: 57 to 69, +6 Life Regen, +177 Max Mana

you should look for a heal with some EE on it
if you just heal and only play cata, a heal charm only is fine

if you want to place arena, you'll need a EE charm + a EE heal charm as acc.charm

Does enchanced effect carry over from one charm to another, like Charm1:50 ee charm2:40 ee
Does my first charm get 90?

no

they're 2 differents charms
and bonuses others then EE don't work on the acc.charm