A couple item build questions.

Why? This comment doesnt really make sense to me......
I thought we established that more levels in X marks allows you to actually hit torrent.

It's probably to do with the fact that it's a Skill shot, like Sunstrike. There's a lot to be said if you can land Torrents without using X Marks the Spot (I can only do it about half of the time ._. ). You might also have an ideal setup to combo the Torrent with, like Banish.

Kunkka + Shadow Demon + Lina == <}3

This is what I meant.You learn landing the Torrent without actually using X Marks the Spot.That way, you'll do it instictively.

In early fighs, it does matter if you torrent deals a measly 120 damage or 300, especially with the boat combo.

If you have setup, sure you could skip it. Sunstrike is easier to land in the way that you use it to finish someone off most of the time. You have setup, torrent on the other hand IS the setup. But there is a much bigger problem with skipping, X marks.
YOU NOW DONT KNOW IF YOU CAN THROW ghost ship if the torrent misses becouse it is mostly based on luck and cant be predicted, (invoker can setup with Cold Snap, you wasted your ult and a lot of mana. If you hesitate on throwing the ship and the torrent hits anyway, you cant throw the ship becouse you will be too late.

And what is more important, nullifing any blink/leap/tp with X. Ensuring the kill on that freefarming carry or snowballing mid? Or 180 magic damage?

What has a bigger chance of happening? The event that torrent misses and the entire gank fails.
Or the enemy escapes on 135 health?

It is better in 90% of situations to max X over torrent.
Mushi puts 3 points into X marks, does that mean he is bad at landing torrents? No, it means that X marks is a more usefull spell.

For your first question, it all really depends on preference as Manofchalk mentioned. Usually if i'm getting assists and not dying during ganks i'd go for the null just for the extra bit of mana, but if i notice i'm being focused alot or dying. I'll go for the bracers.

tl;dr : Bracers = defensive, null talisman = Offensive.