dcs wifi app demos with mike wolf?

I think the reason store demos don't normally include lash ups is the same reason they don't include routes and scenes, etc.  Of course, lash ups are more complicated and require better track signal quality, better wi-fi signal quality, and also require the engines are feature reset so you're starting from a known state.  Bottom line is, it's just a practical matter.  I will try to answer your questions or, at least provide some insight below.

1. The "engine sounds" button is actually not a true "mute" button.  It literally goes out and reads the engines volume, saves it, and then turns the volume down to zero.  When you hit the button again, it turns the volume back up to the prior setting.  This was implemented this way for single engines.  Repeating this process for multiple engines can be problematic and slow in the original DCS Remote.  We agree this is worth looking at to see what improvements can be made in lash ups.
2. See above.  Again, we can take a look at how to improve volume control (and toggling sounds on and off) in lash ups.
3. There is no reason crossing signals cannot be added to the lash up sound list.  I suspect when we did this, not many engines included crossing signal sounds and so, we left it off.  These signal sounds are only played by the lead engines so, it has to be present in order to work.
4. The path to custom lash up lighting, sound, smoke, and other features is as you say, to go into those member engines and make the adjustment.  What you are asking is for the app interface to provide access to each engine in the lash up independently.  While this could be done, this is complex which equals expensive when talking about implementation in the app. 
5. Depending upon wi-fi and track signal quality, etc., moving engines between the active and inactive lists can get wonky.  If there is a repeatable bug, we can squash it but, unfortunately, not enough info here to offer an opinion as to what caused the issue for you.
6. Not sure what is happening with your playable horn(s) on your GP35s in a lash up.  We would have to do some testing here.  Agree it should work.

In terms of your overall experience with the app and lash ups, we allowed the app to get a little stale for a few years there during our transition.  And by stale, I mean in terms of keeping up with Apple and Android operating systems.  The most recent release was brought up to speed to some degree but, there are some work arounds in there.  We are now going back and refactoring both apps (essentially re-writing sections that are older) to make them more stable and reliable.  Also, given we are now shipping the new WTIU hardware, we will be looking into persistent bugs, opportunities to add meaningful features, and improving overall operability in the apps. 

And, for people that prefer the tactile nature of the DCS Remote, why not a new remote that operates on wi-fi? 

We appreciate the feedback.  Stay tuned!

thanks for answering my questions and caring about my issues. glad to see you are still improving the app.