How control appearance of tremolo?

There is no direct way to achieve this. But as often, one can find a workaround by using another voice.
For example: enter in voice 2 a quarter rest and two quarter notes (one high, the second lower, according to the desired angle of the tremolo). This will have the effect of shortening the length of the tremolo, and thus now, add the tremolo sign)
This done, disable the playback (in Inspector) of the notes in voice 2, and hide them, as well as the tremolo sign in voice 1.
result:

NB : it is written on the manuscript “very delicate and discrete”. You understand this about the visual aspect of the tremolo. But perhaps this is just about the dynamics (and playback), another way to indicate a dynamic "pp", as you have already done?
So question: is the game really worth it the candle?

Though you cannot see the stems, they are "there" on whole note. If you put a single tremolo on a whole note you will see which way the "stem" is pointed. Select it a press x and the "stem" will flip like any other note. Cadiz1's idea is on the right track, but the second voice is unnecessary.

Here is a picture of the notes before and after tremolo has been added.