- How may this affect meta-accounters? Will it make pre-decided alliances easier by letting allies come into the game as neigbours?
To some extent, perhaps, but if we're only talking about a couple of accounts, the effect shouldn't be too pronounced, and might be balanced out by other factors.... there are natural disadvantages to choosing Austria, Italy or Turkey, for example, which will make them unattractive to cheats, whereas your chances of actually getting France, Germany or England are reduced by the fact that these are likely to be another players first choice too.... besides, someone who feels the need to cheat probably isn't competent to make choices which will work in their favour.
- Should new players be prompted for their power preferences when they join, or should it be an "advanced" feature, which you need to go to the Settings menu to set?
Prompt them when they join, by all means, but make sure that they can be changed at any time.
- Should players who have no preferences be allowed to join "Power choice" games, and just get last-choice, or should people who haven't set their preferences not see/not be allowed to join the "power choice" games?
Sure they should be allowed to join. They should also be made aware of the nature of the game they are joining, so that noobs don't keep getting stuck with Austria and immediately crushed.
- Is it fairly clear what the feature is about?
Yes.