You can only bane one natural weapon, not all of them, and it can work, but it might be hard to get the CR right.
If you have more then 2 players in your party, dont make the end boss be a single enemy fight. Seriously, action economy means alot in this game. Have it be a family of dragons, or give him some allies. Just dont plunk down a single dragon with or without class levels and expect a good fight. It rarely works.
That said, I think inquisitor might not be a bad way to go, the only issue is how relevant the inquisitor abilities will be compared to the party level. If you are only adding a handful of inquisitor levels, it might not be as worthwhile as advancing the dragon normally. But if you were to take a young dragon and give him say 6 levels of inquisitor, that might be interesting to see.
The dragon should have some sort of backstory about why they're delibrately hunting down and rooting out humanoids.
Adding levels of inquisitor is less powerful than just aging the dragon imo, as mentioned bane will only effect one natural weapon. Your losing bab and sorc spell progression by adding in class levels, and your using smaller hd as well.
If your going to add class levels to a dragon your best options are full progression casters like sorcerer or oracle to give it some powerful spells to match its form or full bab progression classes like fighter for feats, barbarian for rage or natural weapon ranger for favoured enemy human and extra feats.
If your adding class levels to a dragon though it is very hard to keep the cr right, every 4 or 5 cr you want your dragon to be you need to add in an extra class level. So for example a young adult white dragon is cr 9, if your looking to make the dragon cr 15 to challenge a apl 12 party i'd add in 9 class levels.
I'd probably use a young white dragon (cr6) and add in 11 levels of ranger using the natural weapon combat style. I'd give it favoured enemy human +6, animal +2 and dragon +2, favoured terrain cold +4 and underground +2. It gets a few spells to buff up a little and can use instant enemy, it can also use quarry to shadow the party. I'd estimate that to be about cr15, give it double standard cr treasure for its horde, let it use any of the items it has in there that it can use (amulet of mighty fists with human bane if you want to be really mean) and see how it goes. You'll still have to run it well for it to be a good challenge but if you run it really well it could be a tpk, hefty use of ice/snow shaping inside its lair combined with its icewalking ability can make it a very hard fight for an unprepared party. If you need more help i could throw together a stat block for you.
