Have no idea about the MTOW increase, if possible it will be done later in the program I guess.
Anyway the CS100 is offered with 19k, 21k or 23k lbf engines (84.1, 93.4 and 103.6 kN respectively) while the CS300 is only offered with the 21k and 23k lbf engines.

What is the advantage of PW1521 and 1519 over PW1524?

Anyway: comparing the standard CS100 and base CS300:
they have the exact same MTOW (58 967 kg), same wing, and same engines (PW1524G). The difference is fuselage stretch.

Their take-off distance differs: standard CS100 takes off in 1463 m, base CS300 needs 1524 m.
The causes for difference might be:
the fuselage stretch of CS300 causes extra drag during takeoff run;
the maximum coefficient of lift of CS300 is smaller because of lower rotation angle, limited by the stretched tail, and therefore CS300 needs higher speed to lift off.

Is it known how much the takeoff distance of CSeries is stretched by PW1521 and PW1519 engines?

The full MTOW of CS100 is quoted at 58,967 t, not 58,5 t.

My point of MTOW increase was: CS300 has exact same wings as CS100, and also exact same engines (PW1524G, with 103,6 kN thrust each). Yet CS300 manages MTOW of 65,317 t. It would therefore seem to me that if there were any demand to increase CS100 loaded range beyond the standard 5463 km, that could be done with relatively modest modifications. So is there a chance to see a CS100LR, if not with the full 65 t MTOW of CS300, then a more modest MTOW bump?

CSeries has taxied!
http://cseries.com/ftv1s-need-for-speed/

The CSeries, imho, could be stretched to an A320-like seat capacity version without loosing too much performance (with existing wings), perhaps a 26k thrust version of the PW1500G could help in keeping performance equal.

MD-80 had no engines stronger than JT8D-219, with just 21k lb. CSeries has 23,3k lb out of box. So how much bigger could CSeries be than the 172 seats of MD-80?

Looking at the images of CSeries at:
http://www.airliners.net/aviation-forums/general_aviation/read.main/5752656/
post 146
The wing area of CSeries is quoted as 112 square m.
The wing area of MD-80 is also 112 square m, although MD-80 has shorter wingspan and therefore wider chord and lower aspect ratio.

How much can CSeries reasonably be stretched with existing wing and engines?

40% of flight tests still left to do:
http://www.flightglobal.com/news/articles/long-delayed-arj21-certification-set-to-slip-again-386301/

Yes, a pilot operating an international flight operates in airports of the 2 countries connected, and perhaps also the airspace of the countries oveflown (as in this case). And if an obvious criminal is discovered, it is reasonable for any country where he or she is found to detain him or her.

But since the said pilot was defrauding the state of Libya, getting salary in Libya, flying on Libyan flagged planes subject to Libyan civil aviation authorities and breaching Libyan civil aviation regulations, surely he should have been extradited to Libya for trial and punishment?