2. Retain full authority about the buffs and perks the FC uses. Because our FCs are so small, our needs are most likely the same in the FC. However, in the alliance, that might not be the case.
3. Retain the ability to use the housing content as every FC wants.
4. Use the FC chest for purposes every FC decides itself.
5. In the end there is no advantage of merging the FCs. As you said, Linkshells are perfect for organizing and planning events, therefore they offer everything the alliance needs.

The alliance's purpose is solely to simplify the search for raids and crafting/gathering services. We could just ask random people, but we'd like reliable people and getting to know each other.

To be blunt, I'm amazed that such an alliance does not yet exist (and also a bit disappointed that there is no built-in alliance system in the game, but we can work around that with linkshells).