In manhaj as-salikin lesson 4, you stated that if najasa is on the front/back passage then I do wudu, that it is invalid; is this from the fact that it is a condition to do istinja/istijmar before? And what if I woke up without realising i had mathi and did wudu and prayed, is the prayer valid?

Ustadh AbdulAziz al-Haqqan

Yes, because you need to do istinja. If there is madhi, then you prayed with impurity on you. Based on the saying that madhi is impure, the prayer is invalid and you have to repeat it.