How to add or remove instruments in the middle of the score? And how to hide unused instruments? And how to remove the time signature?

Hello everyone! Second time posting.

I'm transcribing a Psalm from two italian composers ("L'anima mia (...)", first page from the .pdf). As you can see from the original score, it starts (Antifona) with organ and a unison in the choir. It then proceeds to the verse (Salmodia), wchich has the four voices harmonizing, without organ.

So, in the Antifona:
- Organ and all the choir (SATB) singing in unison.

In the Salmodia:
- all four voices harmonizing, in this case there's one staff for the men (T/B) and another for the women (S/C).

What I would like to do was a similar thing with my score ("A minha alma tem (...)" .mscz), specifically:

1 - in the Antifona:
____ 1.1 - just one staff for all the voices;

2 - in the Salmodia:
___ 2.1 - no staff for Organ;
___ 2.2 - no time signature;
___ 2.3 - (optional) just two staffs, one for S/C and another for T/B.

I'm incapable of doing these things, and, for the merging of staffs, I've tried out this thread (https://musescore.org/en/node/2986) without success - what happens is that only when NONE of the instruments from a certain bar plays notes does this function (Style > Edit Style > Score and add a check mark next to "Hide Empty Staves".) work...

Are these too many questions? Any help would be appreciated!