on MTBs, they do experience brake fade on their disc brakes... but they do hard braking a lot in short distances... and the reason for their fade experience is due to the same as our car's rotor based brake pads concept that the resin used to bind and compress the metal bits to form a brake pad disintegrates at high heat and forms a gas pocket in between the pad and rotor...

on road bikes, very seldom it is heard that there are fading issues because the entire pad is made from one single material only... the only thing you need to worry about is braking in the wet (especially carbon brake tracks) as that requires a few pulls on the lever to get rid of the water on the pad surface...

Q2. Bike wobble, how and why does it happen ? How to recover from one especially when descending at high speeds ?
you referring to speed wobble? how? this is gonna be long, you gotta google it... other than a bad wheel or lousy frame design...it usually happens when your front wheel hits a bump or hole at speed and the front end starts oscillating as your steering tries to center itself (which it does naturally which is what allows you to ride straight no handed)...

recovery is counter intuitive and depending on the stuation, if only the front end is flinging around... instead of fighting the controls.. you have to calm yourself (hopefully a corner is not coming up at speed) and loosen (not let go, but loosen) your grip and slowly tame it down and regain the controls...

if the entire bike is trying to wiggle itself out, you can use your legs to clamp onto the top tube to stabilize the bike...

there'll be many scenarios..its no point telling you as you won't remember them when it happens as you mostly panic.. it happens fast and this is where your skill for survival and sense of self preservation kicks in...

Q3. When you're in high gear descending and approaching a hill .. the proper thing to do is drop a few gears to be in the correct gear for the climb. Now my question is the cadence up hill is definitely most of the time lower than on the straight .. what if you're in a high gear and climbing it out of the seat and realise you're in too high of a gear .. how do you drop gear in such situations ? sit down .. release tension on pedals .. drop gear .. get out of seat and continue climbing ?

high gear i assume you meant a heavier gear?

in either case, shifting should not be done under high loads or chain tensioned...changing from a heavier gear to lighter gear under loads might worst case result in a delayed and very forced shift... reverse it and you might suffer a chain skip if your chains are not in good conditions... changing from a heavier to lighter gear can be forced as it depends on the cable pull from the lever instead of the derailleur springs, dropping gears is terbaliked...

stay seated if you find yourself in this situations... any botched shifting riding out of saddle has bad consequences... the jolt from the drive train can cause your foot to unhinge from the pedals and introduce your man hood to the top tube if it doesn't send you tumbling to the ground...

it is always good to pay attention to the road in front and watch the gradient, if you think a tough one is coming up... it is better to be 1 or 2 gears lighter than finding yourself over-geared by 1 or 2 cogs to go up the climb...if you are massively over geared when you have to downshift 5 to 6 cogs..time to seriously pay a lot more of attention to your riding...

this is why terrain knowledge is an added advantage if you are riding with your buddies...