Because of the 9,999,999 expereince cap, the ninja can't level more than ~ Lv. 31 and thus have an unarmed AC better than -2.
What is the formula for retaining slots while changing class? In the WA it seemed to me that if you had 9 slots it would become 6 whatever the spell level.

In theory, you should be able to get a higher level Ninja by leveling up a Thief past the Ninja's level cap, then invoking a Thieves' Dagger. One thing, however, is that I don't know what will happen should that Ninja get level drained, as the character would have more XP than the cap after the XP adjustment.

A former spellcaster gets one spell point per spell known. Hence, a former Priest who knows all the spells will have 6 level 5 points, but only 2 level 7 points.

Note that, if you know at least one spell of a level, you can learn the other spells of that level just by leveling up. Hence. ot can be useful to class change a character who failed to learn an important spell such as MAKANITO or MADI.