No direct experience doing this, but I'd take this approach initially:
Each "tier" is a channel entry. You could include things like price, description, etc. here (perhaps this is also a "products" channel used by CartThrob or Store, used to "sell" the tier).
Use an add-on such as Zoo Visitor or Profile:Edit to create members as channel entries, then use a ...
What about Disqus? Personally, I prefer sites that use Disqus for commenting than Facebook comments. Of course, that's not the entirety of your question. In terms of an easy-to-use sign-on. Social Login Pro seems to be the way to go, if you want people to be able to choose, which account they want to sign-in for.
I like that it allows Google and Twitter, ...
Unfortunately this is not possible without hacking the EE core.
If you look at the do_login_as_member() function around line 590 of system/expressionengine/controllers/cp/members.php you will see:
if ($this->session->userdata['group_id'] != 1)
ie, the access restriction of this ...
If Spam through your current comment forms is the issue then there is a great add-on to stop spam in EE called Snaptcha. We were getting a lot of Spam on a site we had built and this STOPPED the spam immeadiately. I can't recomend it enough. Super simple to implement.
We have recently used Disqus on another site we just ...
Haven't done this myself, but did need a sort of multiple "groups" or member "tagging" kind of feature. The easiest way it seems to do this in EE is to expose Channel Categories as a Member field or something similar. There are two commercial add-ons I know of that do this:
IntoEEtive's Member Categories
Addon Bakery's Member Categories.
I chose to ...