لولا: has many meanings, but in this context it means: except for. !كاد يتقدم لولا أن ساق أبي دفعته وأنهار عليه بجملة شتائم...: He was going forward, except for dad's leg that pushed him and then curses flowed on him like a river!

If not : used to say what the situation will be if something does not happen.

he was about to get through but for the fact that my father leage pumped into him

if not