1) Petrol...for a stocko motor i would probably only use standard unleaded with some flashlube (lead additive - which can be bought directly at some servo's or repco etc..)...
Use BP Ultimate if going for premium stuff...I'd stay away from Optimax as it's designed for newer car's with injector's and contains allot of additives for them...but personally with current petrol prices i'd only use standard unleaded unless it's pinging..
2) Ignition warning light, not sure which light your talking about, could be the alternator light or oil pressure light (most likely if your driving very fast around a right hander) though this is unrelated to your gearbox issue..
3) Which brings us to the gear crunch when changing down gears, which is either clutch adjustment, a problem with air or a leak in the system, worn clevis pins or worn synchro's in the box itself.
To change down gear's try double clutching.
When in 3rd and wanting to go down to second. First put the clutch in, pull the gearstick into neutral out of gear, let the clutch fully out..Now blip the throttle to bring the revs in line to what they would be in the lower gear (the idea is to match the gear speed of the lower gear) as you blip the throttle when the revs get around what the would be when in the lower gear, push the clutch back in and select 2nd gear.
If done properly this may prevent you crunching your gears, in which case it would be your synchro's..(engine out job to replace them in the g/box..) Though it could be perfectly fine to drive around with the car like this if you can manage to not crunch the gears, i did this in my first mini for a year..
5) I'm sort of unclear what's happening here, you using the engine to help brake yes? It should not make a difference if you are braking when selecting a lower gear at all, in fact if your any good, you can brake and double clutch at the same time
I have no idea why one of the warning lights would light up when changing down a gear?
6) Are these mph or km/h? If mph they are about spot on for normal usage i would suppose..
You need to listen to your motor and what it's telling you, but generally they sound ok..if km/h your changing to early..
6) Could be you...but i'm betting to say it's either a leaky master or slave cylinder, air in the system, worn out clevis pins, clutch arm..clutch adjustment or dodgy synchros or a combination of some or all of these..