Hello,
1) If you did not change the parent theme files then you won’t lose your changes.
2) a) You can use the Static Block to add the buttons code in 2 columns https://prnt.sc/yp94s6 and show this block in header https://prnt.sc/yp975j
b) Place connection block width buttons in 4 columns instead of 3 to show all the content in one row https://prnt.sc/yp9b60
3) There is no option to add one more HTML block element. Why do you need that? I see that you did not use HTML 1 block https://prnt.sc/yp9mxd So, why do you need one more if you can use it.
What YouTube code do you need to save? Similar to the JS code of the FB button?
Regards