1:
Can you add a parallel battery sled and run two 18650 in parallel; yes it will work;
2:
without any problems?
Errrr... well you have to be carefull never to put the
batteries in the wrong way Ever or you get instant short.
Is it a home made unregulated
mod?
Check the modding forum, and see what others have built; most times people use mosfets to handle the high current.