It's good that you got the report back. Even though it's sad, you now know for sure what you are dealing with.
@rebrascora I'm sure will chime in with tips.
I think you have done a good job by treating the housing with the Virkon. I think that once for now is adequate. You may want to do another clean up in a few months.
Adding more space and trying to keep them as stress free as possible will help too
As for when to switch to layer - that is really up to you. Personally, if I were going to use layer feed, I would wait until all have started laying, but feed is a personal choice. I use a Flock Raiser feed which is usually higher in protein than layer. Limit treats and I would give poultry vitamins every couple of weeks too.
New layers can take some time off, so I would just keep watch on Ruth. If she is active, eating/drinking and being normal, then she's taking a break.
I'm interested to see what others have to say.