I've started replaying this game hoping to finally complete it, and I remembered that some of the "boss blocks" on the overworld can be skipped. At least, it is possible to jump from Deeso to Atlena. Is this useful in any way, or does the game force you to fight all the bosses before finishing it?

As far as I know, the first five bosses are necessary to kill because porting to Afralona requires all 5 strings of the harp which are only gotten from killing all 5 bosses. If there is a way to skip to Afralona early though it would be a big break in the run.

There is a shortcut from Deeso straight to Atlena and that does save time to use but unfortunately you still have to kill the boss in Deeso for the harp string.