Happy to help where I can. Answers in same order as questions
4 exhaust outlets from the head is not unusual on older motors. Picture the valves like this (outlet being the bigger one, obviously):
Oo oO Oo oO Oo oO
and it's obvious what's going on. The middle two outlets are shared by pots 2/3 and 4/5. The outer two are connected to one cylinder each.
It's exactly the same principle as the three inlets.
Get yourself a lower hose with the spring built in. The non-reinforced hoses commonly have this problem. Sounds like the wrong belt was fitted or it's way stretched.
It should have a thermostat. Taking them out is rarely a good idea.
Check you don't have a radiator cap designed for an overflow tank. The correct cap is 7lb pressure, BTW.
The pipe out of the sideplate is an oil-breather, yes.
The sideplate isn't particularly heavy gauge, surely the chrome one you have is just an original sideplate that's been chromed
Re engine number: On what do you base that theory? All original FE and FC motors have a 6 digit engine number that starts with "L". The motors continued without any break from the last FE to the first FC. There's a page
here that explains all.
Please post all the info off the ID plate, as well as the engine no and chassis number and I'll tell you if it all matches and if it's possibly something rare. Standards only came in a very limited range of colours of which there was at least one pale blue.
cheers
RET