You tried to go ALL the way thru the Marble Canyon narrows to where it intercepts the Wacoba/Saline road? (which is what my directions sort of implied) but I think you ended up going the way I intended which ends up on the Eureka/Saline road.
The other route I was talking about climbs out of Squaw Flat west of Andrews Mountain, instead of going out the Marble Canyon drainage then climbing out and over east of Andrews Mnt as you did.
I have heard of someone driving from the Wacoba/Saline Road down into to Eureka Valley in a WW2 truck in the 50's but i dont know if a vehicle (other than maybe a bulldozer) ever went all the way thru the real narrows. Possibly during the lower Marble Canyon mining boom.
We were actually just there March 2 years ago, 5 days of great camping and then woke up to 6 inches of snow!
Here's a snapshot of how far we got down the canyon. The gps tracks are a little scattered I think due to the tall canyon walls and questionable satellite views. In the end, I think I went the way you intended.
Glad that's settled. Now I can move on with life ....