Could improper timing by mechanic have destroyed my engine

My husband took our van in to a local shop to have all the gaskets replaced and to have the van inspected for any other issues (had to replace erg system) in anticipation of a cross country trip. Aside from issues with the valve cover gasket and intake manifold gasket - no mixing of liquids - there were no know issues reported by two other mechanics who had seen it shortly before it was brought to the garage. When my husband picked it up, it sounded off and the mechanic told him it was running hot, but that would resolve within 20 minuets of driving. By the time he got home, (40 minuets) there was an extreme loss of power, horrid clanking and no oil pressure. I could see the bottom pulley was wildly shaking and I found oil coming out of the rear and front seals which stopped when it cooled down. My husband knows nothing about cars so I was the one to see the oil pressure issue and could hear how off the timing was. The mechanic drove to our house two days later and adjusted the timing. He told me he had guessed at it and did not use a light. He then went on to tell me the failure was a coincidence and I need a new engine after paying him $900 for the work already done, including the check over to make sure the van was sound. My cousin, the mechanic who looked her over a couple of weeks before when he replaced a tie rod end (he was the one who told me to have the seals replaced) said the mechanic is likely at fault. However, he is my cousin and is suppose to be in my corner. I would like a second opinion before writing a demand letter to the mechanic who did the work.

Thanks for your time and attention.