A Thief's backstab absolutely stacks with sneak attack damage
Thieves are painful painful creatures, especially with the +3 to accuracy after the attack roll on backstab. Just owie. But you're talking about a scoundrel's interactions with backstab and sneak attack, which presents some complications.
Your scenario doesn't work, but not for the reasons you think
So, backstab is a thief (rogue subclass) encounter power. As a Level 3 thief, you could use a trick to gain CA as a move action, and then backstab + sneak attack two turns in a row.
As a Scoundrel Rogue (normal rogue), you can never have two backstabs. You may spend a feat to replace one of your encounter powers with backstab by taking the Thief's Backstab feat.
You seem to be conflating thief with scoundrel and cherrypicking bits from both. They are different classes, though they pull from the same lists of utilities.
Also, there are better things to do than Preparatory Shot. Many. Many. Many better things to do. (This is a public service announcement.)
The feat, backstabber improves sneak attack, not backstabbing. Ignore the word similarities and move on.
Backstabber, written 3ish years before the essentials line, was a cutesy name for a "more damage!" feat. It just makes sneak attack better. It's a great feat.
Backstab, the utility, is a utility. It isn't sneak attack. Therefore it isn't improved by things that improve sneak attack.
A basis for comparison
Since you seem to like thinking about damage, a thief cheesed for damage at level 3, as taken from here, will:
Round 1, move: use a acrobatic trick or equivalent to gain CA on a target of their choice with a move action
Round 1, standard: Will charge with a vanguard rapier, and the feats surprising charge and backstabber. She will use backstab on the rare event of a miss. This charge does 1d8 (Rapier) + 1d8 (Surprising charge) + 1d8(Vanguard) + 2d8 (Sneak Attack + Backstab feat) + Dexmod + 2 (Bracers of mighty striking).
She will then continue to perform this little trick every round until she gets bored.
A rogue will have a slightly more complex time of things, but can get similar fun things going.
Prepratory shot is a trap. You really do want to be doing weapon damage. And if you actually want to do lots and lots of damage, you want to look at how charge interacts with thief.
As a scoundrel Rogue, and a bugbear, ... and apparently a scholar, if you want lots of damage:
Be a bugbear. Grab gritty sergeant background for rapier proficiency, Be a Brutal Scoundrel so you can add strength mod to all your damage rolls and so you're a Sergeant Brutal Scoundrel Scoundrel (gives you a reason to kill people when they laugh at your description). Attack with piercing strike (attack versus reflex instead of AC makes you significantly more accurate.) Gets you 1[W] (2d6) + Dex + Str + 2d8. Drop the standard "charging chassis" on top of that for extra fun.
On the nature of scoundrels and thieves.
The "essentials" line of D&D books introduced new conceptions of the standard classes. In order to differentiate them, they relabelled most of the classes. Thus, there is no longer such thing as a "Rogue" as Rogue. Instead, there are Thieves (using the rules for essentials) and there are Scoundrels (using the original PHB I rules for rogues.)
They are both considered Rogues, which is why they can take feats labelled for Rogues, and they share utility powers. But a Thief can't take "rogue" encounter powers because her class features give her the Rogue Utility: Backstab, instead. The Scoundrel can't take Backstab because it doesn't have a level: it can only be granted by a class feature.
It is not possible to "hybrid" the scoundrel and the thief. It is possible to fake it with the judicious application of feats, but just like every other hybrid it. Is. a. trap. I've never successfully built a fun and playable hybrid. I've built quite a few unplayable ones though.
When considering classes, consider first your objectives1 and then figure out how the class features satisfy your objectives.
I cannot comment on the cosmetics of the rogue, though I understand no few of them choose to wear rouge, scoundrels and thieves alike.
1 Brian Ballsun-stanton, Samuel Russell (2012) Constrained Optimization in Dungeons and Dragons : A Theory of Requirements Generation for Effective Character Creation.
