1. Sorry, don't know about those other chargers, but if you are in the U.S., you may want to check with the ASUS
U.S. and Canadian eStore site here which doesn't list any chargers for the G750 models yet, but has the 19.5 V DC, 9.23 A, 180w charger used by the G75VW and some other models that will also work for your G750JX.
If necessary, you may want to confirm with eStore that the charger will work for your G750JX notebook before purchasing.
Also, stores selling ASUS Notebooks should be able to order the G750JX charger if they wanted to carry it (not all stores will) and you can find a 'where to buy' at the
ASUS U.S. website here (click on the U.S. or Canadian map) should you decide to look into this more.
2. As far as your Lenovo charger goes, does it only stop charging when the battery charge is above 95%?
You may not be running into this issue, but the battery may not always charge when it's above 95% and you can find more information at this thread
G46 not fully charging battery if you're interested.
Otherwise, you may want to check the output specs of your Lenovo charger to see if it's providing enough power.
Although you need to worry about other things like the AC adapter connector size and polarity of the power pin, if you look in your G750JX battery compartment, you should see a label for the input requirements which should be 19.5V (volts)
and 9.23A (Amps) which works out to 180W (watts) if you multiply the two.
Compare with your Lenovo charger output to see if it has the right voltage and provides enough power.
If your Lenovo charger doesn't provide the 180 watts that your G750JX needs, this could be the reason it's not working that well.
I'm a bit of a worrier, but it's not usually a good idea to continue using the wrong charger especially if you notice the AC adapter getting hotter than normal although the notebook will draw less power if you turn it off while charging the battery.
Well, hope this helps.