Safari in 10.10.4 completely non-functional after simple preferences change Pressed for time?: one simple question at bottom (thanks)

Safari in 10.10.4 has just become non-functional, immediately after I made a simple preferences change to what new windows displayed (I don’t recall exactly what that was, but I was trying to get new windows to display empty and without the sidebar being open).

I restarted twice, and used Disk Utility to do a Permissions Repair and Verify Disk (indicated OK), then restarted again.

I have zero extensions installed; in fact, I had turned off Safari Extensions completely.

Safari opens, but with no window; the spinning beach ball spins indefinitely, and I have no access to any of the top menu.

As Safari’s been acting sporadically odd anyhow since I upgraded from 10.6.8 to 10.8 to 10.10.4, I just installed Safari 9.0 (beta) per suggestion elsewhere here in late 2014;  no change, even after a restart.

No apparent problems except sporadic sluggishness with any other programs I use regularly (Mail, BusyContacts, BusyCal, Pages, TextEdit, Excel).

I had been experiencing sporadic weirdness (not just in Safari) since resolving the disaster of my 24 June upgrade from 10.6.8 > 10.8 (I wanted to stop there for awhile to clean up and export from an ancient contact management program that wouldn't work beyond 10.8), done by the local Apple Store (“We didn’t lose any of your data but we can’t find any of it.”; they found it all, but it took days).

Three days ago, because of such sporadic weirdness, as I usually do, I used Disk Utility to first do a Permissions Repair and then Repair Disk. Repair Disk failed, so I followed the recommendation to repair from the Recovery HD. 

Still some sporadic sluggishness and occasional weirdness system-wide, and some of the symptoms matched those described in Apple’s instructions for reset SMC and reset NYRAM, so I did both (exactly as the instructions indicated, and in proper order).

Perhaps I'm down to one simple question:

I've never done this: What’s the worst that can happen if I re-install the system using the recovery HD (accessed by holding command-R at start-up) immediately after doing 2 Time Machine backups to two separate external hard drives?
I wouldn't expect reinstalling the system using the Recovery HD to cause any significant problems. Minor things that come to mind are reinstallation of fonts in /Library/Fonts if they'd been previously removed, and the possibility of needing to reactivate copy-protected software, e.g. reentering the Microsoft Office serial number. But you've no doubt faced these same issues with the previous upgrades.

You could do a trial run without making any changes to your system:


Use Carbon Copy Cloner to clone your boot drive to a new or empty external drive.
Reinstall OS X on the cloned drive.
If things look good, reinstall on the original drive, or clone back.