1) No. It ticks for the same damage, at the same rate.

2) I believe 3 seconds.

The higher (or rather, lower) damage you see might just be a result of the poison ending abruptly - you basically have to be next to the enemy constantly if you want to keep a constant stream of damage, and in many cases you can't really keep it up on an entire creep wave if you stop moving at all.

1) No it just refreshes the duration.

2) The poison itself does not last a very long time but the poison cloud will linger on the ground and keep refreshing the duration.

mainly its best used as a defensive tool, when attacking targets face to face you are hard pressed to keep your poison on them (unless your running circles around them, which is quite possible as signed) but when running / kiting enemies you can easily keep your poison on them for very long durations.

Between his fast move speed, his goo and fling, poison trail is actually best used as an offensive tool. With fling alone, you can usually guarantee a few hundred damage if the target decides to run in the same direction. But yes, it does play a role as a defensive measure.

Ah, Singed's trail. It scales with AP (not terribly well, but it's a path DOT, so whatev.), it procs rylai's, and it's great for turning the tables from being bait to being a killer.

I believe the duration of singed's poison starts counting down when you leave the poison cloud. The poison effect ticks 4 times I believe, over a period of about 3 or 4 seconds. Late game, this translates to about 400 damage or more with some builds.

I am aware of his fast move speed, goo and fling. which is why I said you actually can run circles around your enemies, but the truth still is that if you are auto attacking face to face your poison is not going on him.

basically as a pure offensive character, almost any other character would out damage singed, since he can't really make use of both his auto attack and ability damage at once. However no other character can kill you by running away from you.

Corki's Valkyrie! ;D

He can, actually. With his ulti or ghost, you can just run ahead of the person and attack them every second, but that doesn't mean he'll out damage a DPS champion. But when you combine doing 80 or so damage per second (was doing 110 just now but you get the point) with his ability to kite and high HP, he can easily out last almost anyone.

Corki, Rammus, Karthus, Nasus, Sion, Amumu and pretty much anyone with a fire and forget AoE damage ability (Morgana, Anivia, etc.) can kill you by running away. Singed is obviously the best at it, however.