1) Root -
https://wiki.project1999.com/Wooly_Spider_Silk_Net (Carry lots of these, for a mage to feel like a wizard / enc in terms of having root all the time you will need a entire bag full 8 or 10) this converts plat especially into roots for you, as you can recharge these root nets at a vendor (use vendors that don't have full lists!!!)
2) It all comes down to timing and getting use to the dmg output of the mob your pet is fighting... melee mobs don't magically just spike out some damage, their max hit is set in stone the only thing that is somewhat random is misses.
Also Remember you don't have to land the new pet as your old one is dying. You can summon the new pet and right before you finish the cast hit the "Go Away" Button on the pet UI window. When you get good at the timing of this you can literally have the new pet almost instantly pop out right as you "de-summon" your previous low health pet.
3) No, pet sit and whatnot doesn't really work, even spamming pet back as soon as the pet gets hit they will retaliate.