the sign-up date means that we get less idlers at the beginning of the game. Because we know exactly when the game starts

Aren't Sign-up date and start date independent. Changing sign-up date shouldn't effect ilders. Changing the start date will.

I would suggest that the people who sign up latest are less likely to be idlers. Those who sign up earliest are more likely to forget or have something else come up...

Just my 2 cents.

I agree, that teleporting is an exploit, and being one, should be blocked. But I am not sure, if diplomacy=4 - because 3=4 in this case ( greetings to my math teachers :D) - is the price I'd like to pay for it.
I am also against changing the rules after game starts, but I accept it when it's a neccessity. And this is the case, in my opinion.
Ultimately, I'd prefer to write teleporting how-to manual, for making the chances more equal, than play single player game with 19 other nations I cannot interact with - what's the difference if they are human- or Ai-controlled, except playing better. I can do it without 19 people. This is not the reason we gathered here, to play single player game.
When there are fixed teams of size greater than one, diplomacy can be restricted, ok, but not when we play teamless game. We should block teleporting with other means, because setting diplomacy to 4, is the one very step before not playing at all - there won't be teleporting too, will it? We play Freeciv, because we preffered to interact with human players over playing original Civilisation I or II with AI players. Playing Freeciv without diplomacy, is like having a car with no starting the engine allowed. Like Freeciv without diplomacy is Civilisation, such a car is just an impractical trailer.
I agree, that teleporting calls for solving it permamently. Unfortunately, my knowledge is too small to produce a patch. I could alter the code properly and safely, but I need a little help to convert the changes into patch-like form. Solution, temporary with no shade of doubt, I see easiest to implement, is changing the function that handles units in case of breaking alliance, for it to recall disbanding procedure instead of relocating procedure. Crude, but effective in means of deterring nasty teleporters. As temporary it may be, it's definitely much more proper solution than writing teleporting manual for newcomers :)
I am aware of many shortcomings of my solution, you do not need to list them in haste :)

Still, I am against changing number of settlers. Both changes in diplomacy and patching ( including 1>move attack patch) can be done without restarting the game.