Engineering Mode (no longer working)

There is a special secret dial code for invoking an extended Engineering Mode on the Oneplus 3:
*#36446337#
This opens up the menue of this extended EM.
It worked. But I screwed it up, so I cannot invoke it any more :(
I know, shame on me and all that stuff, this was totally my fault, as I played around with the apk, copied (not moved) it to the Download folder and tried to install it like a normal app.
Did not work.
But now also invoking the original extended EM no longer works (f@ck).

I'd like to kindly ask the real experts among you:

1. What is it I screwed up with my stupid attempts to install this apk?

2. Is there any way to restore the information the dialer needs for parsing this secret code - other than performing a factory reset?
Or which is the information that got lost or overwritten?

3. All files in

/system/app/EngineeringMode
/system/app/EngSpecialTest

appear to have been unaffected.
(But could anyone please share the proper contents of these two folders, in case the extended EM works properly for him?)

4. Is there any way of invoking the extended EM directly working around doing it with the secret dialer code given above? e.g. by modifying the apk - admittedly in a more sophisticated way I attempted ? :(

I hope my question comes in the proper forum, don't want to cross-post it.

Thanks in advance!

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