1- Yes you can. Player's Handbook, Page 208. Sneak of Shadows feat. This is the cornerstone that makes most Daggermasters work.
Sneak Attack damage. 1/encounter.
2- The majority of damage from Twin Strike comes from Hunter's Quarry and static bonuses to damage such as Iron Armbands of Power, and enhancement bonuses. After those get factored in, the [W] portion of the damage doesn't even register.
Since your argument is that Twin Strikes static damage bonus makes and breaks the build, look at the numbers.
Best case scenario.
+6 damage (Armbands of Power), Magic Dagger +6. Your average non-crit damage w/ Twin Strike w/ Quarry is somewhere around 31 damage. Without, 17. So roughly 48 since you get Quarry 1/round.
+6 damage (Armbands of Power), Magic Waraxe +6. Your Average non-crit damage w/ Twin Strike w/ Quarry is somewhere around 39 damage. Without, 25. So roughly 64/round.
Now shall we factor crit damage?
Dagger - 6 (Armbands), 6 (Enhancement), 8 (2d4 maxed), 14 (Quarry), 21 (6d6 Critical Property). For a grand total of 55.
Waraxe - 6 (Armbands), 6 (Enhancement), 24 (2d12 maxed), 14 (Quarry), 21 (High Crit) and 21 (6d6 Critical Property). Result is 92 .
You will THEORETICALLY crit on average 1/20 attacks with Twin Strike using Waraxes and 3/20 attacks with daggers.
Waraxe total damage through 20 attacks = 1244 (1 crit, 1 miss)
Dagger total damage through 20 attacks = 933 (3 crits, 1 miss)
This math is ASSUMING TYPICAL probability and only 10 Twin Strikes that all HIT (except for one miss). That is the average damage for 20 attack rolls per weapon. With a simulator running constantly daggers theoretically never out damage the waraxe.
If you really want to get kinky you can take a couple Staffs of Knives and get 6d12 critical property, but I'll just take a Vicious Waraxe.
Staff of Knives = 987
Vicious Waraxe = 1262
I hate to completely bust you here, but yes, [W] damage does matter, about 311 damage actually. Especially when you crit. The difference between average 5 with daggers and average 13 with axes is monstrous and 8-24 is just ridiculous. Even when daggers crit, they do not out damage a waraxes normal Twin Strike.
3- He's talking about Two-Weapon Opening, a feat, not Rending Weapon.
One could have easily assumed that's what he meant. Trolls, ftw.
So again, the real issue is not Rangers, but casters. Maxing implement attack damage is a bad bad bad thing. Since it's not based on a [W] damage, but on a static die. Rangers have a better option in Waraxes. Casters, don't.
Critting on anything but a natural 20 is a DnD sin imho. It should be nerfed to, Your attacks with daggers can now crit on 18 and 19 but only gain max WEAPON damage and cannot benifit from Critical Properties or chance on critical effects. Or just change the PP to say max WEAPON damage.