Your operations may well be different.
To answer three of the questions fro OL, 1500' neither significant nor arbitrary and varies from aircraft to aircraft
There is no requirement whatsoever to habitually carry an alternate. Legally, all that is needed is to have 30 minutes of fuel remaining at the end of the landing roll. It may vary slightly, but generally not very much.
In some places there might be perfectly acceptable airports within only a few miles and the alternate is almost in the circuit - Heathrow/Gatwick/Stansted for example. In such cases you can have an alternate, basically without carrying any extra fuel although these are not 'real' alternates as they are almost certain to have exactly the same weather conditions. In contrast, on a Pacific crossing for example, if you take somewhere like Tahiti, which is the only decent runway within a very long way, you cannot reasonably carry an alternate so generally there's some form of remote fuel allowance which might be something like 90 minutes of holding on top of anything else that might be required.
On arriving at destination you could have enough fuel to fly to an alternate 200 miles away, but if held for any length of time that alternate option will disappear. You may still be able to carry a closer airport, but eventually, you'll be left with only one choice. Managing when to hold and when to divert and more importantly if you'll let options disappear, is what we are supposed to to, is it not......?
It's not impossible that you might not be able to consider one place, and so another may be of interest. The option to go to somewhere may exist right to the gate or it could disappear whilst airborne. What's the weather at all of these places. What requirement do they have. There is NO fixed answer and if we play the 'what if' game, then it's easy to come up with ones that are difficult to comply with.