Your first *two* strikes out of stealth/invisibility count as backstabs now, provided you only use autoattack, don't click to attack a second time, and don't switch weapons between the first and second strike (just tested this in-game with two guns, you can't get the first backstab with one gun and then Quick Switch weapons to get a second fast backstab, unfortunately). So two-handed swords are probably better if you want to get in two strikes, unless there's a good way to minimize reloading speed or you can kill with one firearm shot. Looks like arquebus now also has much better damage than pistols.
Cape of the Master Mystic (WM1) grants invisibility when hit by a critical hit, which means 4 backstabs. So maybe a rogue variant of the Thundercat (want to get hit by crit) is possible? That way getting targeted by lots of enemies when you come out of stealth isn't a problem, it's a solution. Nightshroud also gives an extra use of Shadowing Beyond per rest, but you don't need to attack with it once that's unlocked (just have it equipped and switch to it once/rest). So maybe two-handed with Tidefall (great damage, plus healing to make up for being hit), Sanguine Plate (so you have Frenzy for your second set of backstabs), Tempered Helm (lower your deflection after kill), either Steadfast (if you're losing too much heath) or Echoing Misery. Maybe retaliation items with Deep Wounds.
Don't know if Riposte can be reconciled with wanting to get hit by a crit. As I mentioned in the stealth/invisibility thread there are buffs/debuffs that could raise deflection significantly (like paladin's +deflection to a single ally ability)... and that could be great with greatsword. So theoretically perhaps "proc when hit by crit", retaliate, and "then raise deflection high enough for riposte to be worthwhile" could work, at least on lower difficulties. (Of course, an attack either misses, grazes, hits, or crits, so you've got everything covered there.)