Dunno if you're still interested, or if you already had your answers, but Blade Fury is terrible. It does not import
anything. Blade Sentinel and Blade Fury both seem to function like spells; they just do the listed damage, and that's it. They do physical damage. BF doesn't pierce either, but it wouldn't matter if it did.
What's funny though is that when a BS "dies", it scares Fallen as if you killed a monster. Another silly bug.
As for Magic Arrow, it's actually pure physical damage. Not sure if it was intended to be like that, but that's how it is.
It doesn't seem like the damage gets enhanced by anything. For example strength, dexterity, Fanaticism and Concentration didn't increase the damage. +% Damage from items (like the unique Hat) also didn't do anything.
And since you mentioned Attack Rating for MA; if you have an open socket free somewhere, an Eth rune might be an option? It works when put in the off-hand weapon when you're glitchwielding. It might not be necessary though.
You do leech from the damage, but since it's only +19 damage at slvl20, I would just keep it at level 1.
Fun fact: If you use javelins as off-hand when using a (cross)bow, the quantity of the javelin will be used as bolts/arrows. I couldn't really think of a way to exploit this however, because javelins are a poor option for glitchwielding, compared to some of the unique items.
The main pro is that you're not forced to use MA, but on the other hand, javelins don't really get stats that synergize particularly well with Bow Skills, at least not compared to what unique items have to offer. Another downside is that javelins can't have sockets. On top of that, the quantity goes down very rapidly when using (cross)bows..
Lastly, you posted that the IAS from off-hand gets "added" to the weapon you're using, but it does not seem to work like that. Or at least, there's more to it.
I'm under the impression that your base speed will be the average of both weapons (or something along those lines).
For example I compared using a Great Bow, to using a GB bow with a Thunder Maul as off-hand, and my attack speed dropped drastically. Therefore, I bet that you would end up ditching Cranium Basher to Coldsteel Eye because of the crippling slow speed.
I also tried the GB with a Chu-Ko-Nu (-60) as secondary, and it gave a very nice boost in attack speed.
So if you plan on trying out MA with your Buriza, then you could try using a very fast base as off-hand.
Maybe that a regular or magic weapon with a bunch of runes could actually be quite good?
Something like a 4 or 5 socketed bow with some Shaes, maybe an Eth, maybe some Emeralds..
It's probably worth it to figure out how exactly the attack speed thing works for glitchwielding..