Name the script whatever you want and type in the following:
sysval(yoshiflyable)=1
This only works in SMBX 1.4.X as far as I know. This includes versions 1.4.1, 1.4.2, 1.4.3, and 1.4.4.
I have a demonstration of this script on YouTube. Here is the link:
https://www.youtube.com/watch?v= 3BjePgN5YrQ
ANSWERS TO YOUR QUESTIONS:
Question 1: Can you use this script in your episodes?
Answer: You can use this script whenever you want and no credit is needed.
Question 2: Can you do this in SMBX version 2.0?
Answer: Yes you can use this in 2.0, but it is not the same script. SMBX 2.0 uses Lua, 1.4.X uses Teascript.
Question 3: Can you add a sound for Yoshi flutter jumping?
Answer: Yes, but so far I only got it to work on green colored Yoshis. Here is the video:
https://www.youtube.com/watch?v= q8hV0rgqBzY
Question 4: Can you use a different number to make Yoshi flutter jump multiple times before hitting the ground?
Answer: Any other number using sysval(yoshflyable) will still act as 1. So no.
Question 5: Can you make Yoshi flutter jump as a playable character?
Answer: Only in 2.0, and it is a different script used with Lua. Check this video out:
https://www.youtube.com/watch?v= MOy_0p-nT_Y