Answers
There's a lot of stuff here. These don't correspond exactly to the numbers in the question because some numbered questions can be answered quickly. Parts of some numbered questions, however, raise whole new issues. I've done my best.
Retrieving stuff. A creature takes a move action that provokes an attack of opportunity to retrieve a stored item. This is typical for most non-weapons, including splash weapons. Unlike D&D 3.5, the Pathfinder feat Quick Draw specifically excludes alchemical items like most splash weapons (more on those later). I've not played with a Pathfinder GM that differentiates between items stored in a backpack and items stored in a belt pouch or other more-readily-accessible-in-real-life container, but I can imagine some do; discuss with such a GM the house rules that make getting things from a backpack different from getting things from a belt pouch, then, if necessary, buy a backpack, strap it to your belt, and call that backpack a really big belt pouch.
Defending yourself while getting stuff out. Fortunately, characters in many d20 System games like Pathfinder are considered to be largely ambidextrous. If a creature must take a free action to drop a backpack and take a move action that provokes an attack of opportunity to retrieve an item, the creature can continue wielding a 1-handed weapon without issue or take a free action to remove one hand from a 2-handed weapon and continue carrying that 2-handed weapon whilst retrieving. Technically, only actions saying they take 2 hands take 2 hands, even if you'd expect such actions to take 2 hands in real life. A more simulationist GM may demand greater realism of your character, requiring both hands and a dropped backpack, while an even less simulationist GM may not even demand you first drop the backpack! (Note: I don't. Life's hard enough for an adventurer.)
Lighting a torch. Pathfinder makes it pretty complicated to light a torch. Seriously, an adventurer wants to get something better than sputtering sticks on fire as light sources as quickly as possible. Here's how it may go: on turn 1, take a move action that provokes an attack of opportunity to retrieve the torch and take a move action that provokes an attack of opportunity to retrieve the flint and steel; on turn 2, take a full-round action to light the torch with the flint and steel; then either take a free action to drop the flint and steel or on turn 3 take a move action that provokes an attack of opportunity (like sheathing a weapon) to stow the flint and steel. Maybe also on turn 3 either take a move action that provokes an attack of opportunity to get pick up your dropped weapon (as per pick up an item) or take a move action to draw a new weapon.
- Preparing flasks of flaming oil. The rules already cover using lamp oil as a sort of Molotov cocktail. Preparing lamp oil to be thrown is a full-round action that, presumably, can't be done beforehand. Just make a ranged attack made with a splash weapon to throw the flask of prepared lamp oil. Where the cocktail ultimately ends up is determined by the spash weapon rules.
- Throwing weapons in general. A typical thrown weapon has a maximum range of 5 range increments. When you chuck a prepared flask of lamp oil, it's a lot like hurling a flask of alchemist's fire, and that gives the prepared flask of lamp oil a range increment of 10 ft., making the maximum range of the prepared flask of lamp oil 50 ft. (and at such a range the thrower suffers a whopping −8 penalty on his attack roll, although even that severe of a penalty might not matter given how easy it is to hit with a splash weapon).
- Throwing stuff at inanimate objects. The rules for Damaging Objects cover throwing objects at other objects (as well as busting objects with a mace or whatever). It's really hard to break most things by tossing stuff at them. Steel yourself for disappointment.
- Throwing other items. Unless the DM rules otherwise, throwing other items will be like using the items as improvised thrown weapons, allowing most items to tossed about 50 ft. but the thrower starts out suffering a −4 penalty on his attack roll because the item isn't a weapon designed to be thrown. Like real life, most folks just walk over and hand stuff to other people rather than tossing stuff to them, but combat makes things weird, so the GM may have house rules for tossing things to friends (an enormously complicated—and almost impossible—task in Pathfinder; simulating basketball in Pathfinder is really difficult.)
Be aware that using torches and throwing lamp oil are deeply inefficient, low-level strategies. The game makes them available because some folks want that dark, gritty, desperate feel to their low-level games, but by about levels 3 or 4 most adventurers will have gone beyond these and never look back. O, in case it was overlooked, the question's title—about dumping out oil and lighting it on fire—is addressed specifically in the description of lamp oil.
