I think you are adding too much if factor to this. A small home would not require a very big air condtioner any any case so that load would be diversive also. You certainly arent going to have more than one ac unit on the panel anyway. I know what the code says but I dont agree that is necessarily logical when it comes to this specific area. Anyhow just use 2 al and a 90 amp breaker and you are good to go.
If you use the 2 al and a 90 amp breaker you are in compliance. If you put same 2 al on a 100 amp breaker you must meet rules of 310.15(B)(6) or you are not in compliance.
As far as a feeder to a sub in a small house if you did a load calculation on what you connect to it, if it is nothing more than general purpose lighting and receptacle loads you likely do not need a 100 amp feeder to be able to handle the load.
A 60 amp feeder will supply a lot of lights and outlets. If you start to add HVAC, water heaters, or other heavy loads you need to be careful but are often better to feed them directly from the service panel if possible unless there is just one unit in a remote area plus other circuits in that area you wish to feed - that is a good example of when the load diversity may not be enough to justify the smaller feeder conductors allowed in 310.15(B)(6). I don't think the NEC wants you to try to determine what the load diversity will be though -that is something for an engineer to determine. It is not worth paying the engineer his fee to save maybe $20-50 in cost difference of conductors on this feeder.