Dota – Does Windrunners combo of windrun+bkb make her invulnerable

Breaking flaming lasso requires either Batrider or the victim to travel at a speed faster than 500 unit per 0.05 sec. The distance does not matter as long as you break that speed limit.

Exemple : if you use Lasso one someone and the Vengeful Spirit next to him uses Nether Swap on him, the lasso will break since swap is a pseudo blink and thus the swapped unit gets a really fast travel speed

Same would apply with Relocate (Wisp), Glimpse (Disruptor), Recall (Keeper of the Light), Test of Faith (Chen) or any ability that would induce a travel time faster than 500 unit in 0.05sec. In a Similar fashion, If batrider uses Blink dagger, the lasso breaks. Forcestaff pushes a unit forward at the speed of 600 unit in 0.4 sec which is way too slow compared to the speed required to break lasso (500units/0.05sec). So by Forcestaffing, Batrider should never break its lasso.

As for your second interrogation, the damage of Firefly is dealt per second (at max level 80 per second). A "tick" of damage is applied every second and to this number is added a bonus for every Sticky Nappalm stacks on your victim.

Exemple : If you have 2 level 4 stack you will apply 80 + 2*25 = 130 Damage Per Second with Firefly. If you have 3 level 1 stack you will apply 80 + 3*10 = 110 Damage Per Second with Firefly.

Sven's ultimate gives you 100%/150%/200% bonus damage.

Only increases base damage and that given by the primary attribute of Sven. Raw bonus damage is not increased.
– Sven - Dota 2 Wiki

Your damage consists of 2 numbers: A +B, and this description basically means that the ultimate gives you 100%/150%/200% of the A number. It excludes raw damage items (but items that give Strength still count)

Exactly the same can be said about the Double Damage rune: it gives you 100% of A.

Since both of these only increase B and are based only on A, their bonuses will not be multiplied with each other; they're completely independent. But they do stack in a sense. The order of activating them does not make a difference, and it probably wouldn't make a difference even if they stacked in some complicated way.

So if you have both Double Damage and Sven's ultimate level 3 active, your total damage will be:
A+B + A*2 + A (the 2 stands for 200%).