Trying to comprehend your objectives with the stated info. Computer ping pong is an often less than efficient means of productive communication. Suggestions are based on borrowed shop time, scheduling and budget. Expedient production is hindered by any or all of those factors. I'm assuming this car runs and it is not leaving the shop until it is jambed and painted, or at least jambed and assembled. New subframe bushings are required for panel alignment. Let's hope the mounting points don't need repair. Detailing the subframe properly requires engine and frame removal. The fenders are off so now's the time to do at least part of the firewall.
This can get winded. We all know the right way to do the job but your restrictions suggests otherwise. You have to detail the firewall behind the fenders prior to installation. But firewall component stripping now is something you seem to want to avoid unfortunately. Assuming you use satin black and SEM Trim Black or similar you can finish the firewall later, spray then what you cannot now, and it will melt in. BUT, get one speck of overspray on your nicely jambed fenders and it will look HACK. But it can be done if you tape off well. I don't think I have to tell you to prep and shoot the rad support and everything else black you can now before bolting the clip on. You can't skip it. You could put a new blower motor in now, if it grenades later, you'll be busy.
To adhere to your present plans, you can drop the subframe later. I've done it on completed cars, easily. The nose does not need the subframe to stay in alignment. It'll just hang there. Just unbolt it and drop it out. Now do the firewall. Do whatever you want to the frame and slide it right back in.
I should not have to tell you this is not standard practice but your limitations demand alternative measures.