Will a carbon seatpost smooth out any of the harshness in a rigid Aluminum frame or would money be better spent on a certain saddle(recommendations please) or neoprene saddle cover?
I’m no materials engineer but as I understand it, carbon seat posts don’t make any difference due to compression properties. Apparently a titanium one will though. Also get a comfortable saddle.
Depends…on allot of things. My carbon post (Record) did smooth out my Ti frame as compared to my old Thompson…
I can’t tell much of a diff and I have 3 different ones (Campy Record Carbon, USE Alien and Profile Razor - all high end) vs. my old Thompson Elite. They are generally lighter though, but you have to be careful with the seatpost clamp as to not “pinch” a mark in the post, thereby weakening it. My USE Alien has been slipping lately and I over muscled my bolt and have a nice pinch mark to prove it 
The Campy seat post clamp is very nice (light with the offset pincher to minimize this).
I don’t think it’s a matter of compression, as the seatpost is not vertically oriented (it’s at the same angle as your seat tube). The post flexes far more than it compresses. Carbon is a good material for this and depending on the brand (some are better than others) can make a big difference. I notice a big difference with an Easton post, but a negligible difference with an FSA post.
I see the point about the compression which makes me think a saddle would absorb more of the shock than the seatpost. That being the case, which saddles tend to have the most aborption? Would the QR neoprene cover make a difference?
My Thomson Elite gives a smoother ride than anything else I’ve tried, including various carbon seatposts.
It’s strange…but the Thomson’s give a super comfortable ride. I have them on both road and tri bikes, and for whatever reason they are heads and tails above anything else I’ve tried in terms of getting rid of vibrations.