Q4. Who is the mightiest wizard in the land?

I will answer this for the area around Candle Keep and for the whole setting.

The mightiest wizard in the area around Candle Keep (including Greyport) is likely Rajjij. He never leaves his shop, is almost always seen standing behind the counter, and yet his shelves are filled with a constantly changing inventory of magical doo-dads. He claims to be able to enchant items of any kind, though with the caveat that it takes him longer due to the duties of maintaining his shop. And yet for an exorbitant price (to make up for lost sales, of course), he claims to be able to finish the enchantment faster than the laws of magic would dictate. On top of those as-yet-unproven claims, however, Rajjij has demonstrated effortless magical abilities during his sales patter. He is able to cast spells without word or gesture, in fast succession.

Across the greater empires, most would say the mightiest wizard is likely Shenlong, King of Hu. He has mastered magics dating back to the Dragon Empire that the world had otherwise forgotten and has created new spells that broadened scholars' understanding of what mana can accomplish. However, there are other contenders whose feats escape wider notice. The Queen of Enor is said to be an enchantress who can ensorcell one's mind just by learning your name; the King of Asha is known for being reclusive and speaking in riddles but his followers have seen the subtle yet mighty magics he can work; and the unknown ruler of Xochi in truth wields magical power beyond imagining.