wa alaykum as salaam wa rahmatullahi wa barakatuhu!
Thank you for your questions and welcome to the madhhab of Ahl al-Bayt!
1. It is permissible to recite this in both
tashahhuds; although it is preferable to recite the
tashhahud that has been reported from Imams Zayd and al-Haadi, upon them be peace.
بسم الله، والحمد لله والأسماء الحسنى، كلها لله، أشهد أن لا إله إلاالله وحده لا شريك له، وأشهد أن محمدا عبده ورسوله،
2. I only know of the two variations mentioned above.
3. I don't know of any other variations of the salawaat.
4. As for the sitting, the way you mentioned is the sunnah. However, if there is difficulty and pain, you can sit in any way that alleviates such pain. Some of our imams consider the sitting with both legs to the side (like the Malikis) as disliked.
Hopefully this helps! You can refer to the audio lessons on the
Salvationark youtube page
for fiqh classes.
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