At least partially:
METARS are acceptable in lieu of forecast, keep digging for the specific rules when and how. Which probably solves most of your dilemmas.
ALTN airports as filed are not visible to the ATC controllers who are supervising your flight. You can go wherever you like although in their helpful wisdom they might call that field whether or not you will be accepted there. In a similar fashion, for planning: There's no such thing as formal dispatch release document listing the airport you are authorised to use on the day. CMD's responsibility.
Additional interesting knowledge is to review what "TREND type landing forecast" is, I remember too vaguely to type straight away.