Runes of Magic US / AU

pretty much. unless you want to pay for power level/xp orbs...

at lvl 20/20 you get a port to Heffner, which is like self-powerleveling...

mage/warden/warrior is concidered the best combo for mage. But it's still personal preference, try something out and see what you like.

Mage/Scout is also a very popular and fun to play mage, imo. Though since chapter 2 I've always been a Mage/Warden at heart

Imo, M/D would be an excellent class for solo questing, simply because of the healing. You get to heal yourself being a DPS class, which is just... ridiculously awesome. No need to constantly use potions or wait for HP to regenerate.

Goat places - that zone is designed to level your second class. Its a bit slow because the quests there don't give very much XP, but it should do its job and get your druid leveled up. Leveling druid is a bit slow, which is the only drawback, so I'd suggest using mage gear so you can do more DPS. For endgame DPS though, you'd probably want another secondary. M/D is so so for endgame DPS. But that's why you get to choose a third class later on.

Healer and most tank role classes usually are slow to level in comparison to dps role classes. So the d/m, a healer role, is slower than m/d, a dps role.

Something that will speed it up is to do the quest requirements on m/d and then turn in quest on d/m. The trade off is less tp on the d side, but I am guessing that you will go m/blah blah anyways. So more tp on mage side will be beneficial.

I dunno about druid, but leveling my Priest/mage was ok using mage gear with mage staff. I'm guessing it won't be much worse with Druids, but that's just my assumption.

Druids can lvl ok just remember to use mage items and not healer items.