The "tendencies" thing went out with 2e. Maybe 1e.
The 3e portions you quoted where the DM can change alignment are true, but out of context. The context that the DM has to(if following RAW) adhere to are what I posted, which say that alignment isn't a straightjacket and you can act outside of your alignment. If the DM isn't following that context and hasn't clearly stated his house rule before characters are made, he's abusing his authority by ignoring the context that the player has been told by the books is RAW.
The issue is that any semi-realistic personality is going to fall in at a minimum 2, probably 3-4 different alignments on a regular basis. The biggest one is his written alignment. For example, a LG character might also have a gambling habit and love to bet on everything, even to the point of flipping a coin at a split in a corridor to see which way he goes. That consistently chaotic behavior isn't enough to change him from LG into anything else. He still keeps his word, tell the truth, help those in need, speak out against injustice, etc. And you can add other behaviors outside of LG while keeping that core as well.