#1 the ee increases the duration this glyphed item is working, it does not give additional ee to your weapon, armor or charm.
take this as an example:
usually, tier I items will last for 5 minutes. with the ee add, this item lasts a little longer.
Arcane glyph I
Use Item to Receive Effect
Effect Duration: 5.4 minutes
+8% Enhanced Effect
+67 Max Mana
+1 Extra Item Slots
+1% Mind Numb
+1% Critical Flux
+1% Heal Mastery
#2 this depends on your setup to a large extent.
during my climbs with hybrid fighter/samu/pala i went melee till level 30-35 usually and switched to casting then
because i had a 45 int armor and 40 int weapon for this level which easily brought me to 55 ( or even mq )
in your case, you should adapt the level you switch to casting to the gear you have in your own setup, this is very important.
but i wouldnt start casting much earlier than 25-30 because your mana pool is empty too fast.
mana regen gear is extremely important because your pool is quite low, so you will need to regen a lot. good regen gear can therefore speed up your climb significantly.
still, even on lvl 30 your mana pool isnt the biggest. if you have decent melee gear, you might wait with casting till lvl 55 and your mq setup even.
in this area, average melee gear usually looses its speed on chains and casting unleashes its full power.
the int you stat isnt lost during this time when you dont use offensive casts.
in contrast, it will protect you from caster attacks and with a nice healing charm you can heal for a much higher amount while going melee.