Ciao,

ad 1) You can use only those social share icons that are included in Social Share addon.

ad 2) Using Custom CSS tou can override icon, but as I know, there are only two variants of FB icons in Font Awesome.

ad 3) It works only on Mobile device, if somebody has installed WhatApp application.

as alternative solution, on Joomla Extension Director find Social Share module that fit your needs, and use it, using Module addon. >> https://extensions.joomla.org/instant-search/?jed_live%5Bquery%5D=social%20share

Sorry but probably i dont be hope.

2. I need to change the post image of Facebook

https://pasqualemarcellini.it//forum/social.jpg

Many thanks

ad 2) Do we talk about Article or Page?

If article, the best way would be to install OG extension that allows you to choose image used in Facebook each time. Otherwise Fb takes the first image that see it in sourcecode.