I don't believe that's how surprise works.
Normally, characters are surprised on a roll of 1 or 2. The number of segments a party is surprised for is represented by the number rolled. For example, if the bad guy rolls a 2, he is surprised for 2 rounds. If the bad guy rolls a 1, he's surprised for 1 round.
No. You subtract the difference in the rolls. To be surprised at all, the bad guy - absent special circumstances - has to roll a 1 or 2. To not also be surprised, the thief has to roll a 3 or above. If he rolls a 6 and the bad guy rolls a one, that's 5 segments.
It's almost impossible to surprise a bad guy for 5 segments. Elves and halflings traveling alone and in non-metal armor can surprise on a 1-4, but that's about it.
\Actually, they surprise at 90% or 9/10.
Note also that encounter distance for surprise starts at range. 5" - 10" indoors and 6" - 24" outdoors. You never start at backstab distance in the beginning of surprise.
Not always. A thief hiding behind a door can surprise when the bad guy walks through the door.
It also takes surprise segments to move. You can move 1' per 1" of movement rate in a segment so it would take at least one surprise segment to move into a backstab position.
Not always. A good thief sets up situations to his advantage.
Finally, while it is true that you can take a full round of melee attacks each segment of surprise, it is debatable whether dual wielding grants backstab status for both attacks. Unless I'm mistaken, neither the PH nor the DMG rules one way or the other and thus is left up to the DM.
1e DMG, p. 62.
Because the party surprised is (relatively) inactive, the surprising party will be able to attempt telling blows during each segment of surprise as if the segment were an entire round! That is, a fighter able to attack twice during a normal round of combat will be able to do so twice during each surprise segment.
This is pretty self explanatory. It doesn't specifically say you can dual wield during a surprise round. But it doesn't specifically say humans can attack in a surprise round either. Or dwarves. Or women. Does that mean they can't?
Theoretically, I can see 3 backstabs (or 6 if you're allowed dual wielding backstab) if you're playing an elf or halfling thief, but the star would have to line up perfectly.
The table in the DMs guide look's like this:
Surprise Dice Difference Lost Segments
0 .............................................................. 0
1 (2-1,3-2, etc) .......................................... 1
2 (3-1, 4-2, etc) ......................................... 2
3 (4-1,5-2,etc) ........................................... 3
Note, there is no 6 in the table, even though it is a possible result. I take that to mean the table should be extrapolated to 6-2, 6-1, etc. But even so, 3 lost segments is still a crapload of damage. That's what makes a thief such a badass. He was never intended to engage in a straight up fight. He opens the fight and takes one bad guy completely out, or SEVERELY injures the boss. Then he just help out as he can. He's probably made himself a target, so withdrawing is probably a pretty good idea.
And that's what the 5e rogue should look like. The sneaky guy who can climb, find and remove traps, pick locks/pockets, decipher languages, be the scout that takes out the guard, and able to hold his own in a general melee.