Thanks in advance for your help.

Heres some backstory on the car
-94 civic with d16. Got it as a project and have fixed every issue (wrong pullys, replaced the junk ebay coilovers, new valve cover gasket, etc).
-I had a loss of power from a stop and it ended up being a clogged cat. We removed the cat and welded in a straight pipe.
-replaced distributor, plugs, wires, map sensor, main relay to solve some starting issues where it would crank but not start. Once we used a paperclip to check codes it started right up. We let it run and its been starting ever since.
-checked IACV and the FIACV or whatever thats called. the FIACV was loose so I tighted that back down. The IACV looks brand new so I assume the previous owner replaced it to try and solve this issue. I took it off and the mesh screen looked perfect.
-Today I tried to bleed the cooling system but my heater didn't work. I'm in FL so not much use for a heater so didn't notice until now. The lower radiator hose never got hot.

I assume since the lower radiator hose wasn't getting hot there was no way for me to properly bleed the cooling system but curious as to what I need to look at or replace next.
Also curious if anyone knows why this rev issue is only in park and neutral and if that is a symptom that will help me diagnose this issue.
I'm going to go drive the car around for a while now and make sure it doesn't act differently in drive at red lights.

 

The reason this only happens in park and neutral is because that's when the engine isn't under any load. A new IACV makes me think that that's still your issue. Typically these are replaced with aftermarket junk. If you can find a good, used OEM IACV, that would be much better.

Take off your air intake, crank the car, and let it idle. When the idle starts to hunt, place your finger over the IACV port inside the throttle body (see image below). If the surging idle stops, the IACV is still your issue.