Lot's of things to talk about. First, what do you mean you had to change to a zero offset post to get the fit right. The seat position fit is independent of the reach to the bars. If you changed the post to shorten the reach to the bars, that was all wrong. You only change to seat position to affect the position of your hips and legs over the pedals. I'm starting to wonder if the bike is not simply too big for you, and you got sold a bad deal.

Second, a long, deep channel can help with saddle comfort but pressure on the perineum suggests your saddle may be too narrow. If the sit bones in your butt are not being supported (hanging over the edges of the saddle) all you weight is resting on your perineum. You may need a wider saddle. As I said, channels or cutouts can be valuable to fine tune comfort, but only after you get the right width.

Finally, if the bike is squirrely for any reason, do not ride it. I have experience with the speed (or aka "death") wobble up front at 35 mph downhill, and I assure you it is no fun. Luckily I was wearing brown shorts.

The more your write, the less I think you got the right bike. Check out your saddle position with a standard measure like KOPS (knee over pedal spindle). Look it up if you aren't familiar with it. No this isn't absolutely right for everyone, but it isn't so wrong either. If you are way in front of that in order to reach the bars, that bike is absolutely too big for you and needs to go back to the shop for an exchange.

Tell us some about your measurements and the bike size. I smell a rat.