I had to change my code for 18.4. I patched it using a sheet instead , it would also fail the first time it was shown, but then on subsequent display the sheet would stay up and users would be able to interact properly with the sheet....... 😩

Yup, looks like it is fixed in 18.5.

I downloaded manually, as suggested. The manual install took a while too but went well. It did not help with the other ongoing install. Even after a reboot. I searched a bit and found suggestion deleting some ~/Library/Caches files may help. I deleted files in relation to store (app store). Rebooted. Launch Pad was showing an empty progress bar for a short while and then disappear. Now XCode shows as "GET" on the app store. But now, at least, installd is not eating up 120% CPU... thanks.