How to Increase the Mileage of RX 100?

The best I got was 42kpl and the worst was 30kpl

But for 42 kpl, I did not go above 45 kph

to get a somewhat decent kmpl (30-35 maybe?) 

That is what u can expect for a properly tuned bike. Ensure the following points are taken care of:

1. Carb Jets are stock (115 main and 20 idling)

2. Float pin and Jet Needle not damaged

3. Good quality air filter. Please try and source OE (Yellow) which is around Rs 300, but really good 

Tyres:

Which tyre sizes from which brand are known to provide optimum fuel efficiency for the RX100?

Stock Tyre sizes on the RX100 are 2.50X18 front and 2.75X18 Rear. I would recommend 3.00X18 for the rear for better road grip

The tyres on my bike are worn out and it seems the stock numbers are not easily available. Although if anyone can share information about where to procure them in Mumbai (preferably near Mulund) I would really appreciate that.

More than Tyre condition, it is the inflation pressures involved that count. Check air pressure BEFORE your journey ... AFTER the trip, the heated up tyres will give you incorrect readings

Pressures for RX100 are:
Front 22 PSI and rear 28-32 PSI

Tyres are available rather easily for the RX100 ... Try Diamond Tyres in Chembur, near RK studio ... he has a good selection of tread patterns

My mechanic told be it would cost around Rs. 3000 to get both changed.

The estimate is correct

Fuel Tank:
I would like to know what is the exact usable reserve tank capacity of the RX100 ('93 Model)
10.5 ltrs capacity, 1 ltr reserve

Are there any bad implications of running the bike on reserve for a long time?
If u run the bike on reserve, the grime accumulated at the bottom of the tank will get into the carb and give you trouble

I usually fill petrol in denominations of 50 or 100.
Should I be filling a greater amount of fuel and run it more through the main tank?
And what are the implications, if any of the above queries on fuel efficiency.

Keep the fuel tank as full as possible. This displaces air from the tank and ensures that the evaporation losses are minimal and since there is less air in the tank, the condensation of atmospheric water is less ensuring that the tank does not corrode from the inside.
PS. ensure that u put in one 40 ml pouch of 2T oil in the tank whwn it is FULL to aid in the stopping of corrosion

Iridium Spark Plugs, Performance Air filters:
Any implications (good or bad) on fuel efficiency.
I guess performance parts would generally decrease the kmpl, so can anyone shed some light on this and by how much do they affect the fuel efficiency.

Stock works best

RDD CDI Kit:
Also I would like to know about the RDD CDI kit

No experience with this one ... although u are right about saying that performance mods will decrease FE

Any other general fuel efficiency tips for the RX100:

Ride in a controlled manner (I know that is difficult for an RX100 rider LOL)

One thing that I could recommend for FE is installing the 5-speed bike's 5th gear ONLY, not the entire gearbox

The top gear ratio in the RX100 is 23:23 ie 1:1

The RX135 5-speed 5th gear is 22:24 ie 0.91

It will cost around Rs 3.5 k for the conversion