1. This is true for a character with +1 LA or +2 LA (which is the majority of characters who actually use this feature), but not for all characters.

2. Depends on the DM. I agree that allowing a level 15 character to buyoff a +2 LA with 18000 xp would be ridiculous. In that case, I would reccomend that a character could buyoff the LA, but at the cost of their current ECL being used.

3. I was under the impression that having two sources which give a character +1 LA makes a +2 LA character. I don't see anything in UA's Reduced LA section that contradicts this.

4. This is a very good point. Level Adjustments are really a bad idea to use in the first place. Unless you're using one of a handful of very good +1 LA templates or races, then the cost is rarely worth it. However, if you do pick from one of those great templates or races (Feral, Mineral Warrior, Half-Ogre, etc.) then the cost at low-levels will be easily tolerable, while the cost at a higher level (say 15+) would be terrible.

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You are right in that the LA Buyoff system is broken, but I think that stems from the fact that the LA system was never supposed to be very functional in the first place. It seems like it was designed as an intentional, "Don't pick this race!" sign that later mutated into some viable builds for lower levels.