Hi quietman,
Speaking as a scorer in the 99% on the verbal GRE, as well as someone who has taken a fair amount of LSAT practice tests, I can assure you the GRE is exceedingly learnable (both V + Q). Also, it's much easier than the LSAT overall.
I can't speak to the efficacy nor accuracy of the Manhattan diagnostics (I used Magoosh), but I can attest to RC on the LSAT being much more difficult than the verbal portions of the GRE.
To hit 170, basically all I did was practice all the Magoosh problems and memorize (or attempt to, anyway) 1,000 GRE practice words. I'm assuming someone who scored -3 (is that a freaking 180? ) on the LSAT probably already has a pretty firm grasp of the English language.
I wouldn't be disheartened were I you; I'd be encouraged. Anyone who has an HLS admit in their back pocket (yes, I stalked you a bit) shouldn't feel humbled by the GRE!
I'm sure your score will improve the more you practice. In fact, I'd bet on it.
Also, not to give you unwanted advice (but, of course, why else would you be here?), but I'd really give due consideration to the HLS/HKS joint degree. I read your other post where you stated you were more interested in MPP-specific work than law, but I'd also point out that although the Kennedy School is very good, it hasn't produced presidents (and countless other senators, CEOs, billionaires, etc.) yet. I know it's crazy expensive, but you're holding one very golden ticket (I'm sure you already knew that, though).
Anyhow, good luck with the GRE verbal prep. I have faith in you!