Thanks @Louis_Frolio  ,

To be precise

1. It's not generally constrained, just for spots, as removing spots but with the same instance type works

2.  it's a very small cluster, so no point in autoscaling, if it can't take 2 workers, then it won't take more ..
3. Driver is already on demand

4. Generally I think it's a bug with strange behavior, my guess it would work if the job was already running and the instance is evicted, but something isn't working correctly if spots aren't available on "cluster start" 

5. I did see this behavior in AWS with 15.4 so I don't think it's DBR version issue, again, I think it's a core bug in the behavior . but if you can't point me to where this was fixed I'd be glad, maybe, I'm mis interperting what I saw with15.4

I'll read about fleets and multiple zones, maybe it can help