The Break DCs don't make a lot of sense. The DC for breaking manacles is 26, and 28 for masterwork manacles, but only 24 for bending iron bars. I have broken chains before (when pulling things, by accident) but I couldn't imagine bending the kind of iron bar used in storm grates, prison cells, etc.
That said, if you want to stay consistent with the current DCs, you could go with:
Break 1/2" rope: DC 21
Break 1" rope: DC 23
Break 1.5" rope: DC 25
Break 2" rope: DC 27
Add +2 for any masterwork ropes.
Remember that 2" rope weighs 4 times as much as 1" rope. I don't think it's the kind of stuff anyone would be attaching to a harpoon.
Not yet in combat
Rogue casts invisibility, draws dagger.
Guard fails all Perception checks, and is unaware of rogue.
Just before combat
Rogue decides to attack, combat begins, initiative is rolled. (Rogue 8, Guard 5)
Init 8: Rogue moves stealthily towards guard.
Init 5: Guard surprised, doesn't notice the rogue.
First Full Round
Init 8: Rogue makes full attack.
Init 5: Guard may act, and is no longer flat-footed.
Everyone is assuming that rogue is better off if he skips attacking in the surprise round so that he gets his full attack. Guess what, he still gets the full attack even if he attacks in the surprise round. You don't have to argue about whether he is ALLOWED to skip his attack, because there is no reason to.