These are all really helpful and insightful suggestions. This community is amazing and I'm very humbled by how helpful [and sometimes funny] you all are.
My update is thus: The car is back to starting on first attempt but doesn't on the second (ignition off, ignition on and cranking but no life). I don't dare drive it for fear of breaking down so these tests are done on the street outside my house: I disconnected the Engine temperature sensor (black horizontal plug near the cylinder head) and the car started every single time that I started and shut her down. Whether it was for 2 seconds of running or 10 minutes. This went on for about 30 minutes of intermittent/random test intervals just to be sure it was the engine sensor causing the problem. Then it wouldn't start again (starter motor going but not starting engine) just as if the engine temp sensor had been re-connected (it hadn't). I googled some info about how an engine temp sensor works and someone on Quora explained that when disconnected the ecu gives a best guess of how much fuel to inject. The user then suggested trying a little throttle when cranking. Sure enough the car roared into life and I could smell fuel (and probably damaged cat converter) but the engine was alive!
Turned off the engine and re-connected the temp sensor (engine was very hot) and predictably it wouldn't start. Tried giving it some gas and the engine once again started without problem.
I think my mechanic's guess [replace the engine temp sensor] was right. What do you think? Is this conclusive proof that I need a new engine temp sensor?
Sorry if that sounds thrifty but I'm unemployed atm so i'm trying to avoid any unnecessary part purchases
P.s. I had a cheap wired ORB2 wired in for the duration. It gave me a p0115 "engine temperature sensor 1 circuit" error but then about 15 minutes into testing (before I plugged the sensor back in) the error disappeared, without me clearing it, and I couldn't get the ecm to give me the error again despite exiting the program and trying to scan again. Not sure if this is useful info but through I'd include it.