Regarding the walidity of tawassul.. I believe in the ahl-e-sunnah there are certain schools of jurisprudence which are believed correct and ijma beyond their directives is closed (correct?).
Four references I can find demonstrating that Sunnis knocking on tawassul are going against their Tariqa's stated principles.
Imam Malik:
The Abbasid caliph, Mansur al-Dawaniqi, once asked Imam Malik ibn Anas whether he should turn towards the grave of the Prophet (s) or face the Qiblah for supplication? Malik answered him:
Why do you want to turn away from the Prophet (s) when he (Prophet Muhammad (s)) is the wasilah (means) for you and for your father Adam, towards Allah on the Day of Resurrection. Turn to him (the Prophet) and seek his intercession (shafa'at). [Taken from Al-Mughni ma' al-Sharh, vol. 3, p. 588; al-Sharh al-Kabir ma' al-Mughni, vol. 3, p. 494; al-Qadi `Iyad in al-Shifa
(2:92-93)
Imam Hambal:
Imam of Ahl’ul Sunnah Ala' al-Din al-Mardawi said in his book al-Insaf fi marifat al-rajih min al-khilaf ala madhhab al-Imam al-mubajjal Ahmad ibn Hanbal (3:456):
“The correct position of the [Hanbali] madhhab is that it is permissible in one's supplication (dua) to use as means a pious person, and it is said that it is desirable (mustahabb).
Imam Shafii:
The Family of the Prophet are my means and my intermediary to him. Through them I hope to be given my record with the right hand tomorrow”.
Ibn Hajr Haytami, in Al-Sawaiq al-muhriqa li ahl al-dalal wa al-zandaqa (eg page 180) AND in al-Khayrat al-hisan (p. 69) This has also been found in Diwan al-Shafii as edited by `Umar Faruq al-Dabbagh (Beirut: Dar al-arqam, n.d ) p. 50.
Imam Ghazali
Imam of Ahl’ul Sunnah Abdul Hamid Ghazzali allotted a special section in his book Ihya' 'Ulum al-Din concerning the manners
of pilgrimage to the shrine of the Prophet (s) in order to repent and seek forgiveness from Allah. He stated:
The Prophet should be made the means (wasilah) and the intercessor (shafi'), and with face turned towards the tomb, the pilgrim should implore Allah for the sake and position of the Prophet with the words: "O Allah, indeed You have said, Had they, who had wronged themselves, come to you and asked Allah's forgiveness and the Apostle had asked forgiveness for them, they would have certainly found Allah
Most-Propitious, Most-Merciful (Holy Qur'an 4:64);
O Allah, surely we have heard Your words and we obey Your command, by coming to Your Prophet to seek his intercession with You for our sins; how burdensome and heavy (are sins) on our backs! We repent of slipperiness, we confess our wrongs and our faults, accept our repentance for his sake, make Your Prophet intercessor for us, and exalt us for the sake of his position and his rights with You."
I've taken these references from an excellent article I found on this, but its too lengthy to post in toto. Anyone interested plz contact me for it.