how to keep the driver door from closing on an uphill grade?

Gravity assist? Why would you want the doors open?

 

Assuming the car is still on the originals, getting new door stops/hinges will help a lot. The driver's door on my old G2 was very worn out and was always subject to the the winds.

 

The term you might be looking for is "door checks". They are separate from the hinges. If you open the door you can see them, black plastic rods sticking out from the front edge of the door and attaching to the body.

When you are on level ground, do you feel two easily-noticeable rest positions for the door, one about half open and one fully open?

If you don't feel those positions distinctly, the checks could be wearing out.

I'm not sure if they ever would have held the door on a 15% grade though.

 

If those linkages got greased at some point, they’ll be less able to hold at the detent points.

What’s the miles on the car? Is the passenger side on same slope, and does it hold? If that’s the case, could be driver’s side is worn and you could try replacing it.
It’s not too difficult a DIY, had to do this decades ago on a car.

Other’n that: use your foot?

 

Sorry to say this (because I'm cheap and lazy), but LHD I'd use my left arm, RHD I'd use my right arm. If I'd need both for control I'd probably use my head...

REVVL V+ 5G ?

 

A search of either term will result in the correct part.

 

In my Gen 1, I actually had to replace the hinges on the driver's door, some time after 200,000 miles. That was for a different problem (you could wiggle the end of the open door up and down because of the worn hinges, and that was enough to make horrible body-flex noises over uneven roads).

I found those parts by searching for the term 'hinges'.

Even if a search engine shows you both the checks and the hinges when you search for either term, you still need to know which will solve the problem you're having.

 

Don't recall needing to remove door panels, at least on our string of Hondas. Looking at our 2010 fwiw, the bolts are on the outside, easily accessible. For next time.