Need some help !!!!!!!!!!!!!!!!!!!!!!!!!

If you had residual pressure in the cooling system, then I would not be immediately be suspecting any sort of crack in the the heads or block, or a head gasket issue. I can't see any of those leaving residual pressure.

Buy/rent/borrow a cooling system pressure gauge and cycle the engine to hot and look for pressure pulsations in the coolant pressure, at idle fast, idle and while revving it mildly.

If you have oil in the coolant, it should be obvious as a scummy substance. If the coolant is clear, I would not suspect oil. Have you actually wiped the brown/black gunk off of the t'stat housing area and felt it and smelt it? Is there ANYTHING in the coolant in the rad? I am almost inclined to suspect residual fine rusty material and other crud left in the cooling passages from the block cleaning process in the shop. Seeping out of the t'stat gasket like that and drying might look like that. And the coloration does not strike me as oil, especially form an engine with new oil with a just a few miles.