Use of percussion on Sonatina font...

Since the Sonatina font does not have its own instrument-list.xml file, you may not know where to find it.

Percussion instruments are most likely in a kit.
in SSO-Complete.sf2 version, tam-tam is here: //in other versions, god knows where is :)
Percussion-Cymbal => keys : 57, 59, 60 (a3, b3, c4 {middle C} )
Percussion-All Percussion => keys : 67, 68, 69 (g4, g#4, a4)

You cannot use an unidentified percussion instrument on a single line. But you can create it by defining a drumset or single instrument.

for sf2 version:
Sonatina is a non-GM compatible soundfont.
Since the sonatina is not GM-compatible, it will not work properly with the defined instrument list. And timbres not compatible (the preset numbers are located in different places) with other soundfonts.

1. I agree
2. You must use the default percussion clef
3. Perhaps it's called gong or chinese gong? In a test file add a tam-tam to an instrument and see what it is called in the mixer. If it's not like a gong, your sound font may not have one. You will be forced to use another sound font that does have a tam-tam. you can have more that one sound font loaded. MuseScore will use the first sound font listed in the synthesizer first. So make sure the second sound font is moved to the bottom of the list.
4. Yes, as long as you follow the instructions properly.