It was a long trudge but, in the end, I used the yay --editmenu option to edit PKGBUILD and remove both patch downloads and the patching of two files.
yay --editmenu -S torbrowser-launcherI also needed to
sudo pacman -S base-devel and install option “[5]” to eliminate a “fakeroot binary missing” error during the build. (See Fakeroot binary missing).
git was previously installed and is required too.
It’s all moot though, because once built, torbrowser-launcher runs and promptly downloads the x86-64 torbrowser, runs it and generates an “SSE2 required” error message.
It seems that Flatpak, Arch, and yay all want users to waste hours of time rather than just making architectural compatibilities and requirements readily known! 
I will try antman’s suggested torbrowser build next.