Which are the only ones I should get as a pure sniper?

I was thinking:
Shield Wall 1/5 (Because I need it as a prerequisite)
Power Bash
Deadly Bash
Disarming Bash

Another question I have is,

Did Bethesda leave out blocking with bows on purpose? Or will it be patched so that you can in the future? If so then I may want to take quick reflexes and more into Shield Wall

Get elemental block too(Or whatever reduces magic damage by 50% while blocking). Very helpful.

shield charge is one of the most powerful combat abilities in the game, especially for n archer who may get swarmed by melee opponents from time to time.

I would say skip all of the blocking, unless you plan on skipping stealth. Otherwise with stagger, paralyze and the other archery perks, nothing will get close to you.

Ah I didn't notice pure archer only.

I'd say block is completely unnecessary. The benefits are almost exclusively shield based. The bashing is nice, but it does such little damage and doesn't stun all that long.

You can't actually block with a bow right now, and that elemental block perk only works for shields, not 2H weapons either

Power bash doesn't work with bows, and you can only disarm with a power bash. Deadly bash *works*, but even fully capped out with a good bow the bash damage is negligable (around 30 damage with daedric)

I concur with a few posts here -- you have too many tools at your disposal already to need a shield.