OK, I'm looking at the photo on the left. I'm looking at the throttle arm. Behind the throttle arm the photo shows a small area of clear firewall. That is a simple line of sight, clear of everything, clear of muffler, clear of crankcase, clear of engine mount. This little area is "around the corner" of the engine mount from where you currently have the pushrod coming out. That's the spot I would choose, just from looking at the photo.

It may be that if I had the whole thing in front of me, I might find another spot I would like better, but the spot I'm speaking of looks like it should work. Complications which are unseen in the photo, such as what's behind that spot on the other side of the firewall, might need to be dealt with. Perhaps a shifting of the fuel tank. Perhaps another hole drilled through a fuselage former. This might call for a long drill bit from the hardware store, a 12" bit perhaps. If that's what you need, buy it, because you'll use it more than once. They don't cost much.

To plug the unused hole in the firewall, just cut off a short length of 1/8" dowel, or whatever size your hole is, tap it into the hole in the firewall and hit it with a drop of CA to seal it, or cover it with a dab of 5-minute epoxy. (By the way, if your firewall is fuel-proofed with epoxy, you should sand the surface before you try to epoxy over it. Most epoxies have a wax component that comes to the surface during cure. If you try to epoxy over cured epoxy, the wax will impair the bond. So just sand it lightly and the new epoxy will bond to the old.)

I understand your frustration with what seems like it should be a simple matter. I've been there many times. Throttle pushrod routing takes imaginative thinking. Such things are never spoken of in the glowing reviews of ARFs and kits, because they're too darned tedious. Don't be fooled. All those reviewers have on more than one occasion sat helplessly and furiously cursing a model in front of them. I've had firewalls with more holes in them than swiss cheese.