Brother, thanks for your questions, listen carefully,....
first of all I ask you to read Dua of Tawassul in Arabic , in your own language. Your questions are about Tawwasul(Asking prophets and Imams to ask God), why dont you find your answers from our books? If you had studied that Dua, you would have never asked many of those questions.
That's a famous peace of writing which we read basically once a weak. In that Dua we first ask God, then ask Muhammad, pbuh, to ask God then we ask Fatimah,s.a and the successors of Muhammad,pbuh to ask God.
You ask me why dont we ask Muhammad,pbuh first?!?!?!?
I also apply Tawassul by asking from Jesus,pbuh and Noah,pbuh.
It was just a couple of months ago, that the husband of my sister said 'I saw Jesus,pbuh in my dream, he,pbuh, opened a locked door'. My sister said while she was surprised 'Yedterday I took Tawassul by Jesus,pbuh'.
I dont wanna say that dream was true or not, or Jesus,pbuh, really helped or... I just wanna let you know that it's not just Ali,a.s. and Husain,a.s., from whom we ask.
Second of all, the crying is out of Love. When I cry on their graves it's because I like them. I wanna be with them. Dont you sigh that you cant see or watch them?!
Dont you wanna spend some time talking to Muhammad,Jesus, Ali, Mahdi,....,peace be upon them all?
Third of all, I just wonder how you say that they are dead and they dont hear us!!!!!
There are many authentic Hadiths about it!!!
Dont you say 'Assalumu alayka ayyuhan Nabiyyu wa rahmatullah....'?
If no, then you have missed his,pbuh sweat answer.
In fact we are the dead, they are alive.
Infact we are slep, they are awake....
Forth of all, we also ask God directly. Dont we?!
We have many peaces of writings, Dua and prayers which help us directly ask from God and only a couple of Duas to ask inderectly. Check out Dua of Komeil by Ali,a.s. the first successor of Muhammad,pbuh, Duas of Sahifa Al-Sajjadiya, by Sajjad,a.s. the forth successor of Muhammad,pbuh, Joshan Al-Kabir and hundereds of Duas in which we ask God directly.
I suggest you to study Dua of Komeil, in which Ali,a.s. teaches Komeil to say these sentences
'O God, forgive me the sins which doesnt let my prayers be accepted'
Which means there are some sins which will block our prayers snd requests for a certain period of time, in that case, we can use Tawassul.
'O Muhammad, my sins might have blocked my prayers and suolications, you ask God to forgive me, as you are Wajeeh'
Dua of Tawassul
http://www.duas.org/tawassul.htm
Dua of Kumail
http://www.duas.org/kumayl.htm