You make one critical wrong assumption.
The ways of tareeqat are not developed after the visaal of the Prophet (SAWW) by sheiks and ashaba...
No, that would be innovation.
During his life, while the Prophet (SAWW) was establishing a muslim estate in Medina and completing the tenets of Islamic sharia, at the same time he was imparting the ways of tareeqat (love and closeness to Allah Ta'alla) to his closest sahaba. Abu Bakr, Umar and Ali (Radhi Allah anha) are on the forefront... but there are some other major sahaba too.
If we check the lineage of all silsilas of tareeqat, most of them converge at Abu Bakr (RA).
For centuries the silsilas of tareeqat only got imparted from person to person. And this is really the only way. These days for additional info books are now available, but without a teacher it is a risk not worth taking.
Re: your second question... Mansur Al Hallaj was a cursed man. He was great Aalim, but in the throes of his jazb, he cried out "I am Allah". Listening to this, the islamic rulers ordered him killed. Thats all what I know about him.
Bayazib Bistami is a learned scholar. Whatever I have read about him makes him a great scholar. I am not sure if you can group him in the category which is conveniently labled 'sufi'.
I have not heard about the others.
By the way, the spiritual connection between man and Allah Talla is confirmed from Quran. I did had lots of books on this, but not here. I will try to search for links on the internet, but really don't expect to find much there. If I can ask someone to bring those books to me, I will be able to provide more answers.
In any case, the ways of tareeqat are not for everyone. For the most part, sharia is all you need. If you have such grave doubts, then tareeqat is definitely not for you, atleast not till you have a really good teacher. So, all that you should do is concentrate on gaining knoweldge about islam and sharia...and try to live your life according to the sunnah you find in the books of Islam.
In the mean time, just try to be humble in your approach, because there are many things you don't know, and these things I can't explain to you here at this point.
PS. Your last question, The Naqshbandiya, named after Baha al-Din Naqshband (d. 791/1389) is a tariqah that is widely active throughout the world. It is one of the many facets of this topic.