OK, I did it just now. It works. Not that easy.
First, I think it is easier if you open the access panel from the wheel well. Just turn the wheel opposite to give access to the panel. Pop out the push pin by pulling the upper part of the pin back, and then the pin just pops out. The access panel can then be pushed out from inside the engine compartment. Note that it is riveted to the bottom of the access port, so don't wreck it. Just push it open hinged there. It gives plenty of room, and much easier to see.
Open the light housing by opening its access port bayonet style. Twist counterclockwise to release the bulb and socket assembly bayonet style. Remove the bulb from the socket. That leaves you kind of a large socket with the attachment housing still attached. No need to take that off. Plug in the new bulb from its pigtail. Negative goes up. Test it to make sure it lights. If not, reverse the plug.
Take your assembled new H7 adapter and plastic adapter which have been assembled per the instructions. Yes, the new plastic piece stretches over to fit. Put it on a finger, and working from both the engine compartment and the access hole to see it, illuminate with a flashlight, and attempt to get the thing to fit into the light housing. The larger flange goes down. I agree that the fit is tricky, and on the driver side in my car (left) I had to sand the tabs of the adapter a bit. Just be patient. You kinda have to get the lower flange in tipped forward a bit first to get the others to fit. What ever you do, don't drop the assembly into the lamp housing. Don't ask me how I know. But if you are stupid enough to do that, and can't fetch it out, open up the bulb housing for the turn signal more forward, and that gives access to that section of the housing. You'll see the damn part sitting there. Knock it back and fetch it.
Once the adapter is in, first feed the bulky plug inboard of the lamp holder inside the housing, or you won't be able to get the cover back on. The bulky thing does fit in there just fine. Now bayonet your new bulb into place, with the pigtail cord pointed down to orient the bulb elements at 3 and 9. Test. Put on cover and don't forget to close the wheel well access port. You're done.
Not night yet here, so I don't know if they will be better, but I will report back. Success so far.