Sent Back the Pro-ject Turntable: What would be the best option in the $1800 range?

Get the GR. Stick it on your credit card. You won't notice in a year but you'll be happy you did. 

@rego Don’t you mean the GR? The G is $4k. 
Even if OP gets the 1500, he/she will be wanting/regretting not getting the GR.

@klimt Is this deja vu? Dude, you're still not getting it? A $200 cartridge on the GR will sound better than an $800 one on that Debut.

To buy today on Amazon with free returns:
Bright and clear - AT VM540ML (stretch to the 740 or 760)
Musical all rounder - Ortofon MC3Turbo
More body, a little warmer and musical - Nagaoka MP300 (stretch to the 500)
Be warned. All of these require 20 hours to even start sounding their best so don't jump to conclusions.

Do I need to get a Schiit Mani Phono Preamp for MC and MM Cartridges?

IIRC, you have an old receiver. Yes, you should get the Mani for now. The phono input in the receiver will not do justice to the new table. Speaking of which, at the very start, I recommended you get the Mani before dumping the Pro-Ject. 

Probably okay. But how do you know it didn't fall causing a possible alignment issue, or been out on demo(s)?  I'd be inclined to buy it new.

@klimt You better come back and let us all know how it goes!

I second the Polytable (with Jelco 850) as an alternative to the GR.