1- There are over 400 million Shias around the world, and it’s natural that you’ll millions of them to be ignorant or misinformed or extreme. You’re right, there are Shias who have questionable beliefs.
2- Regarding do sujud by a grave, here’s our perspective on it:
1) If one does sujud to the grave with the intention of worshipping the grave or the person buried in the grave, that’s kufr and shirk, and such a person is not Shia and not even Muslim.
2) If a person does sujud towards a grave with the intention of respecting the grave only, not worshipping it. Meaning the intention is to respect the person because he was a good servant of Allah…that’s not kufr or shirk. Proof? In the Qur’an, in several chapters, Allah commands the angels (who are His pure creation) to do sujud to Adam. The Qur’an uses the word sujud. All angels prostrate except Iblis. He refuses and he fails his test. Now God didn’t command the angels to commit kufr or shirk. That’s impossible. He commanded them to show respect to Adam (because He chose Adam to be his khalifa), and so the angels worshipped God, but through Adam. Meaning Adam was the gate to God. They had to show him respect to show their obedience to God.
If a person does this type of sujud to the Prophet or Imams because Allah chose them as His khalifas, that’s not kufr. However, is it appropriate for us to do that? Scholars are split:
Some say yes, just like the angels did sujud, we can also do sujud with the intention of respect.
Many say no, it’s not appropriate. Let’s keep sujud only and only for Allah, and we don’t have evidence from the Prophet or Imams that they asked people to do sujud by their graves. Yes we have hadiths telling us to show respect to the grave of the Prophet and Imams, but not to do sujud. So it’s not appropriate to do such sujud, but if someone does it, it’s not kufr. It’s not shirk. It’s just something inappropriate.