Hmm... the ECU being "opened" sounds ominous. There's all sorts of mischief that could have been going on in there.
Anyhow... I would look at the lambada first.
You can get a good brand (e.g. NTK) on fleabay for not tooo much.. and if it doesn't solve the problem then at least you have a spare sensor. Apart from the catalyst, nothing else
The lambaba circuit at 4.x volts sounds right.. I remember reading something (I'm not leccy-techie unfortunately) that the signal drops to 1.x volts when it's doing it's thing.. so if it's stuck around 4v and not changing, then something isn't happening..
<Hopes someone can explain it better>
So... first check the lamda relay. There is a relay that controls the lambda heater and the signal circuit... if the connection is shagged, then there's your problem... the lambo won't work.
Your pattern part cat might not be great quality (have a look what brand it is - Bosal? Timax etc. will be as good as anything) but don't get hung up on that. It's new, so it will work... even if that's "just for now". It only has rare-metal honeycomb in it.. quite difficult for it to suddenly fail.
If you still don't get anywhere, take the lid off the engine and one at a time, remove the coils and examine the end-connectors. All that fiddling about may have damaged a corroded end.. either into the coil or into the spark plug boot.
While you're there, peek into the plug wells and make sure they're not full of oil, though if the garage had the rocker cover off, then it should be okay.
Take the plugs out and make sure they're in good condition. I'm always depressed how often garages just put crappy plugs back in..
If you get desperate, take the rocker cover off and turn the beast over by hand until you get TDC on #1 cylinder (closest to the cam-belt) and check the timing. If it's out, you could get wonky running even if everything else is fine.
Your other questiones... Don't worry about the oil. If they used any 10W40 semi-synthetic then it's good enough for your old donkey. Alfa's like oil.. but they're not oil quality fussy. The minimum spec' is impossible to buy these days. Any 10W40 semi-synthetic is good enough.
Don't worry about the key. If it starts the car then it works and has nothing to do with the problem. From memory, the flashing light on the dashboard means that the car thinks that the signal from the key (alarm button) is weak and your battery needs replacing. I presume your Alfa key was a red one with the rubbery alarm button built into it. Your Fiat one might be similar.. ? but stick a new battery in there and see if that sorts out the flasher.