*Edit*
Check out the WIKI...you can find most info you were asking on there.
http://wiki.project1999.com/Shaman
Here is the Shaman section...if you scroll down past spells it gives some info on pets
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*end edit*
Buffs do affect pet...including quickness etc.
I believe when summoning a pet they can be between a level range...ie your level 34 summon will summon at 28 to 31 lvl pet...something like that.
No you can not summon two pets.
Most effective solo would be Root rot and keep pet for xtra damage.
Cant answer your Root vs Enstill question yet.