Removing the old paint “edges”

Most of what you are saying is why I was suggesting finding a good body shop and or bodyman/painter that actually has experience in collision and or restoration.
Can you post a picture of the can of paint you used to paint the engine bay? It would help to know what you are using.
I have been using PPG Omni single stage MAE Acrylic Enamel to do under the hood/doorjambs/trunk of the 74 Dart Sport I am working on so that it would have a somewhat factory look to it as the rest of the car is original paint. No sanding or buffing. Plus if your not happy with the shine level, you can color sand it and clear it.
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I stick with products I know and don’t really mix manufacturers when it comes to solvent/primer/paint. And if you do have something unknown you are trying to just go back over with little prep (it’s not advised) seal the areas with a really good primer sealer.
One more thing, stop where the cowl meets the upper plenum on the firewall.
The cowl should always be sprayed/cleared with the rest of the body so that the color matches.
When doing something like your car I like to paint the jambs/under hood/trunk then hang and mask off all of the fresh painted areas and shoot the outside of the car as one.
Single stage or base/clear it will all be even this way.
If you do decide to paint everything on stands then I would do the cowl and engine compartment along with the rest of the body.

It really sucks when you don’t have a good Jobber to rely on to give you the right advice. Sounds like the guy you talked to is used to DIY guys painting tractors.
Let’s see what you are working with and you might be ok...