There is no keen bow that is viable DPS in end game that an archer would ever want to use. LitII, Alchemical Earth/Air, Epic Bow of Earth or Epic Thornlord are your choices.In particular, the Epic Thornlord will get significantly greater effect from Improved Critical and probably remains the highest DPS bow in the game. In addition, at cap you will be focusing more on the CC aspects that IPS brings to the table and Improved Critical has very nice synergy with shimmering arrowhead to apply the crippling effect.
Boss fortification in challenging content is now lower than what it was before. For epic lob, for instance, once the following effects are applied fortification will be 0: weaken construct (blasting chime), destruction, improved destruction (once they are fixed to stack in U13), improved sunder (even my rogue can do this in epic) and a double stack of FvS aura.
Even against 100% fort foes IC:R still adds DPS in the form of bursts (bursting and blasting on LitII for instance).
All 10k star archer builds have manyshot as well. The only builds that might have 10k stars and not have manyshot are melee builds who take 10k stars for a little ranged DPS on the side. 10k star archer builds have other perks to balance out the DPS like kensei specialisation, ranger favoured enemies/rams might or earth stance III enhanced critical modifier. Not having any of these and not being able to fill a healer spot means that you will have to try very hard to compete with these builds and missing Improved Critical: Ranged is going to be a big dampener on your attempts to do so.
On 1 hand a 13 cleric / 6 monk / 1 wizard build can plausibly fill a healer spot in easy epics or as a backup healer with 3 mass cures and 2 feats to back it up. For this cleric build you can get away with gimping your DPS a little as you can bring something besides ranged DPS to the table.
On the other, a FvS build only has ranged DPS and self healing - basically something that any 10k stars build can provide. You need to optimise your ranged DPS and improved critical is a big part of that.
This makes the argument for IC:R all the stronger (and kind of screws up your previous point).
Sorry for the miscommunication, I use the terms weapon proficiency/focus/specialisation as I can't be bothered to remember the intimate details of the relationship a FvS has with it's diety.
Blade Barrier is not going to see as much use end game as what you seem to think it will. Anytime that another caster w/BB is around you'll have to put your blade barriers away as in all likelyhood theirs are stronger and they don't stack. Additionally, the actual DPS of the blade barriers is kind of marginal compared to melee DPS so you have to make sure that your kiting isn't preventing melee characters in the party from connecting (or you'll be reducing party DPS).
The DC will be ~3 lower than a similar caster. Unless you have 2-3 sorcerer past lives? If you don't have empower along with maximise and CL:15 (achieved with an epic abishai set) then your blade barriers will be quite mediocre and definitely trumped by a wisdom based cleric or favoured soul or an artificer.
Another point is that cleric can access Cleric Wisdom III for another possible point of DC/more 10k stars love (depending on gear). Not sure if this is possible with enhancements though.
I assumed you'd done your research.
I don't see any heavy domination in areas that actually matter.