Observable.FromAsync supplied with Scheduler vs specifying .ObserveOn(Scheduler) (or both)

[sabgut]

I have several asynchronous subscriptions in the UI that I'm using Observable.FromAsync on to deal with concurrency. One thing that I can't quite figure out is relating to the overload of FromAsync that contains the scheduler.

Lets say that I have an async method:

private async Task _AsyncMethod(object t)
{
_SyncUpdate(t);
await _DoAsync(t).ConfigureAwait(true);
}

That contains a sync part _SyncUpdate that I want to update the UI with and an async part _DoAsync that is awaited (can be run on any thread). I set up my subscriptions as follows:

observable
.ObserveOn(UIScheduler)
.Select(t => Observable.FromAsync(async () => await _AsyncMethod(t)))
        .Concat()
        .Subscribe();

Here are the questions:

1. Will ObserveOn ensure that the synchronous part of _AsyncMethod is run on the supplied UIScheduler, while _DoAsync can run on whichever other thread?
2. Or is it necessary to use the overload of FromAsync that contains the scheduler as well to make sure the synchronous part is run on the UI thread, such that the code should look like this?

observable
.ObserveOn(UIScheduler)
.Select(t => Observable.FromAsync(async () => await _AsyncMethod(t), UIScheduler))
        .Concat()
        .Subscribe();

3. Or should synchronous parts never be run inside Observable.FromAsync, so it only contains asynchronous calls?
4. Finally, if _DoAsync is awaited with ConfigureAwait(true) (explicitly written here for clarity), will it return to the synchronization context specified by the scheduler after the async part finishes?