Skill points are in fact what determine your advancement in skills, not XP.
When you gain XP (usually by killing a monster, but also sometimes through a potion of experience), you gain a corresponding number of skill points which are then distributed amongst your active skills. The ratio of skill points to XP changes with the total XP (or, analogously, the total skill points) that your character has ever gained. At the beginning of the game, the ratio will be roughly 1:10, but it will gradually decrease until it hits a cap of 265:10, something which will almost always be reached by XL27.
Players will notice the effects of the decreasing ratio of skill points to XP, even if they don't realize it. Early on, killing a difficult monster like a hill giant may raise a L0 skill several levels. Later on, players will be lucky to get even one level out of killing the same hill giant, even if they still only have one L0 skill activated. This effect is also particularly notable when quaffing potions of experience - they give considerably more skill levels early on.
One very important fact to keep in mind is this: ignoring cross-training, the total XP required to get a chosen set of skills to a chosen level is invariant. That is, if you decide that your MiBe will go for L27 Axes, L27 Armour, and L15 Fighting, it does not matter if you train all three skills to L15 first and then train the remaining two to L27, or if you train them one at a time - at least for the purposes of total XP investment.
Absolutely none. Skill training routes do not affect the total XP required to gain skills. The fact that it is easier to gain skill levels when total XP is lower might suggest that training few skills early on is good strategy, but this is already considered optimal anyway. The old learndb entry for skill points was "Only galehar [a developer] should know or care what these are." It may not have been the most illuminating, but it is correct in its message: skill points are something the player has no control over, and cannot use to their advantage.
The skill point ratio is actually determined by looking at the XL a character with a 130 XP aptitude in the old system (partway between 0 and -1 in the new system) would be at. The skill point costs in XP are as follows:
|XP 130 "level"||Skill point cost|