Chance Wade's page

If you want a Dex build then you might consider taking advantage of the Elven racial weapons.

Mindblades have proficiency with all martial weapons so an Elf with the Weapon Familiarity Racial Trait or a Half-Elf with the Weapon Familiarity Alternate Racial Trait consider all of the following as racial weapons and would have proficiency with all of them.
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Wow, that's actually a really cool idea. Basically you'd make the character into a master of elven weaponry, which has some really cool thematic potential. It's not what I originally envisioned but now I'm definitely considering it.

This could actually be a hook to get the party involved with the mayor of Magnimar, which is something that happens eventually, anyway. Maybe the mayor finds out what happened with the impersonations and makes some sort of agreement to make the charges go away if the PC's keep quiet.

That's an excellent point, actually. The book describes the Lord-Mayor as being legendarily corrupt and "exceptionally quick-witted", so I could certainly see him waving the charges if the PCs keep quiet about the impostor and agree to investigate the murders for him (which they would have done anyways, just now with no reward). Plus I could use this opportunity to introduce Ironbriar as Kalindlara recommended.

First, you kinda f#+~ed up as the DM, it's writen that they do switch back to their original form once they think the PCs are on their trail or when they're ready to kill the PCs.

Ah, I missed that line, you are correct. Although, I don't know why they would change back anyways...