Using Cat5 for DSL from NID - good idea to double up wires for signal integrity / strength?
Given the possible interference would it be worth it to double up cables from the NID to the router (i.e. use blue/blwh together for ring and orange/orwh together for tip)?


No. 

Besides what asndre said, which is true, if you put two transmission lines in parallel, the following occurs:


Impedance is halved, so it will be mismatched.
Since one line will always be a little bit longer than the other, the signal at the end will be the sum of both, therefore it will include two copies of the original signal, summed with a slight delay, which will degrade signal integrity.


In other words, use good quality outdoors-proof waterproof twisted pair telephone cable, with the same impedance as the original phone cable. It'll work fine. The less connection boxes on the way, the better.

If you think you might someday need to put a connection box somewhere on the cable, simply reserve a meter extra cable at this point, coiled. If you use conduits (you should) then keep a bit of extra of cable at the end that you can pull through the conduit if you need to cut it in the middle to install an extra connection box.