Several different needle sizes are contemplated down to a 36 gauge. This will enable filling canals as small as size 15. In other words, the filling method no longer determines the size of the prep. Cleanliness is the only requirement. Curved canals are no different than straight ones.
Cement flows around curves.
There is no need to get the needle to the end of the canal. Any kind of sealer can be used but the current favorite is a ZOE based cement. If a post is going to be used, it is a simple matter to fill to the bottom of a pre-prepared post space. Completed canals are indistinguishable from other filling materials, except that most dentists find themselves making much smaller preps. If the filling is short it is simple to re-insert the needle, add some cement at the coronal end pushing the cement to the apex.