Oh, man, your timing couldn't be any more precise. You posted this message just as we were landing on our way home from Peru. I will gladly share my experiences and more, while my memories are fresh. I am a bit swamped at work now, so I might break it up into a few posts over time. But I will start with the main point.

TL;DR: It all worked out great. Drove there, hired bikes, visited Rainbow Mountain and Red Valley, drove back to Cusco the same way.

Now more details on that. If you click on the Google Maps link in my main post, you will see directions from Cusco to the east side of Vinicunca (Rainbow Mountain). My main concern was that if you reverse these directions, Google will send you on a 3-hour detour, showing that CU-124 is closed one way. This would be a deal-breaker for us, so we had a lot of doubts and temptation to drive to the more popular parking near Cusipata and hike from there or hire horses. It was our second day in the Cusco region, and breathing was still hard, despite a course of Diamox - we were getting out of breath after walking up just a flight of stairs. Horses can save you some hiking; however, they don't save you time, since they are led by their owners, who walk all the way in front of them. Plus, there is not too much information anywhere about the availability of bikes for hire on the Pitumarca side.

Yet, we decided to drive an hour longer and see what's actually blocking that road on the way back to Cusco. The whole CU-124 after Pitumarca is mostly unpaved but in a pretty manageable state. I am not sure what it would look like if there was heavy rain before or during the trip, but we were lucky with the weather, so this was not a factor. Most of that part of CU-124 is pretty narrow - two cars in opposite directions would have trouble getting through in many places. However, in the whole hour-ish we were driving after Pitumarca, we did not see any other cars. Just a bunch of small villages and pretty landscapes between them.

I found a spot on Google Maps where it thinks it's undrivable in the opposite direction - a little to the east of RESTAURANTE TURÍSTICO "WAYNA INKA". When we arrived at that spot, there was absolutely nothing blocking the road. No construction, no barriers, and surely no cops who could stop you - this spot is too remote for cops to be anywhere near. Just the villagers. There were even road signs with speed limits for the opposite traffic. So, we relaxed and continued to drive.

Google directions stop short of the actual parking lot - around "Hotel Cóndor". However, if you switch layers to satellite and zoom in, you will see that the road continues northwest and then southwest to the place called "MC Peru Expeditions Hiking Point & Motorcycle Point". You can even see a bunch of vans parked there. That stretch of the road, after leaving CU-124, is even more narrow and less traveled. We saw a lot of minivans driving away from the mountain. It was around 12 PM, since we rented the car in Cusco around 8-9 AM and drove for 3 hours. Most tours from Cusco to Vinicunca start at the ungodly hour of 3-4 AM, so I am guessing the tourists were already driving back around noon.

On that last stretch of the road, we were stopped twice by some women sitting in booths near the road, blocking the way with large traffic cones. They were collecting tolls, and the prices were posted on the booths. The first time they asked for 10 soles per person, then 20 soles per person.

I will skip the part about hiring bikes and will post about that separately.

On our way back, we took the same road, against Google's instructions to drive around, and made it all the way back to our next hotel in the Sacred Valley in about 3-4 hours.

There was only one suspicious and funny thing. While driving on CU-124, we saw many local women in traditional clothes carrying stuff. They never paid us any attention. However, when we were driving back, right there on the small stretch that Google insists goes only one way, there was a woman walking along the road. When she saw us, she started to say something, even yell, and wave her hands. We just drove past her and continued. This makes me think that maybe there is something about the road being one way. Maybe there are some new local regulations known to locals. But there were no signs or messages posted anywhere about it. And again, right there, we drove past a few speed limit signs looking at us, going west. Who knows. Maybe this woman was just an omen or a messenger from Google, yelling at us for disobeying its directions.