I don't know anything about IOS but I broadcast my HR (Bluetooth HRM) to the Tacx app on Windows no probs, and I'm 90% sure that I have done the same to the Tacx app on Android (but I haven't used it on Android for a long time, so I might have mis-remembered).
As to merging things, sorry I can't help.
I've pondered this some more, if you're using the Fenix watch as a HRM in broadcast mode then I
think that's ANT+ only. I
think you have to put it into special "broadcast only" mode, so it becomes just a HRM. And I
think that this is only ANT+. I'm pretty sure - because I've read about it - that this isn't supported on IOS. (You'd need an ANT+ dongle for Windows - not that that's any help).
Which is all just to say that I think you're probably right about not being able to Broadcast the HR from the Fenix to Tacx on IOS.
But at the cost of a bluetooth HRM you should be able to. (I
think 
)
And I still have no idea about your merging question. I strongly doubt that there's an easy way other than downloading the TCX files and running them through some special software.
