b.t.w. - I did a small test in Truespace where I arranged 120 fighters as seen in the FS mainhalls on how terran hangarbays look, and compared that to hte Orion. Guess what - they take a helluva lot of space. Far more than even I tought! 
What on earth makes you think that's how they're stored? The mainhalls seen in FS1/2 are quite probably launch bays or staging areas for scramble ready or mission prepped fighters, not actual storage of those held in reserve.
The Galatea and Bastion main halls are supposed to be fighterbas on destroyers.. And incidently, they are arranged rather well, in the shelf-like system. However, as evident fro mthe mainhalls, there is a lot of free space between the individual fighter holds, increasing the hangarbay size well above what I though it would be.
Easily? Then why haven't they. Give me one good reason why, if it was feasible, the GTVA doesn't have that sort of close-to infinite range weaponry. Even assuming there aren't limits placed by physics restricting the damage-per-energy factor of weaponry. Or, hell, if they do, what makes you think it won't have, y'know, a logical increase in power requirements or heat build-up?
simple - becoause [V] can't account for every invention, every weapons and every ship ever created by terrans. Look at it realisticly - how many fighter classes do we have today in service? How many ship classes? Heck, mroe than you and I can count.
And somehow, in FS 2 the whole terran fleet has the "vast" diversity of 2 destroyers, 3 cruisers and a dozen fighter/bombers.
If they were to maek things real, we would have 1000 differnt ships...but then again, who would ever model or invent all of them?
the question is not "does it currently exist in FS2" but rather, does FS2 permit it to exist?
All of which place compromises upon the design. You move your reactors under armour and deeper inside the ship, and you have to place more cooling around them (for crew safety), put in countermeasures against an overload and explosion taking out the heart of the ship, place more power conduits from those reactors, allocate the same space to backups, etc - not to mention the problems if you have to service the things externally and they're buried under several feet of metal. You can't just plonk down a bunch of fusion reactors without consequences to the ship design.
A good desing takes al t he things you mentioned in account. And when did you become a starship building expert? As far as I nkow, most things about FS starhsip is unknown or vauge - the rest are all assumptions.
Going by the Fenris, maybe you could decrease the Orions' speed after bolting on some extra armour. Otherwise it's the same ship, slightly different specialisation.
AS I said before, the Fenris didn't have anything you can take out to put new stuff in. an Orion has - fighterbays and al lthat comes along with them. That would clear up a lot of room for far more improvments than compared to the Leviathan.
If you want good node defense your capship have to be in range to deliver the hurt. Blockading the node focuses on crippling/destroying enemy warship as fast as possible, before they can launch fighters or bring more reinforcements trough the node.
You're assuming the battleship will take longer to destroy, but in actuality - like any FS2 capital ship - all you need is a few Helios hits and the thing is doomed. and also relying on - yep - destroyers to provide it support
And you're honestly telling me you would send a single ship to break a blockade? If a fleet is parked on the other side of hte node you will send a fleet to counter it. Otherwise ti would be suicide no matter what ship class you send in.
So support is a must - and note - I never said a battleship should be without any fighters whatsoever (even the Iowa has a halipad). It would carry 2 interceptors wings or so, to bolster it's bomber defense.
And you've still not given an example of the armour, turrets, etc of such a vessel.
For a design example I would use my Archangel (only scaled down aand with the firepower scaled down accordingly..after all, I did design it with post FS2 era in mind, so it does have more a bang-per-buck than FS2 would allow).
2.5 x the hitpoints of a Orion
8 main turrets, positioned on the top and below, 4 beam cannons on the sides, and another undefined anti-cap weapon below....and the rest is pretty much point-defense weaponry. Extreemly strong subsystems and main weapons. Ability to concetrate at least 60% of it's anti-cap firepower at any point in space.
2 small fighterbays with a squad of interceptors each.