but Vlemish only gives 15 mana / second, that's not that much

+ Plenor gives 700 mana more than Vlemish

What are the other good mana items available, for example if Im planin an infinite mist build for LE, I need more than Velmish and Pleanor. So what other mana items I can buy, of course that are realistic in a balanced game ?

It gives 525 more mana, but produces about 12 more mana per second. 525/12 = 43.75 seconds before you make back the raw mana boost. Not that long of a time. In almost every situation, Vlemish is better.

So what about #1 & #2?

Yeodan:

1.  I play generals 99.9% of the time so i don't know.

2.  well the obvious ones: Banded, nimoth, hauberk, groffling (i like to use these 4 for my oak builds)

then there's unbreakable, godplate.  Bloodstone & Narmoths (the rings are only good if you have high autoattack speeds)

There there's:  Girdle of the giants, bulwark & all fathers ring.

3.  Vlemish is better up front (usually) becuase it provides regen whereas the plenor doesn't provide much regen at all unless you already have vlemish. 

One thing to point out though is that on Leviathon, the dynamics change since there is a +mana regen flag on the map.  This means that Plenor is the best one to get first.  For some demi's just getting plenor is enough for this map.

you are right, the first question was not fully answered and i must admitt i only observed it being triggered by AA, no matter what demigod. as there are favor items with effects that can be triggered also off specials i don't know for sure if this may be here the case too.