Both occur at the same time ("When hit"/Step 10) and thus may be declared in either order.

This statement is not correct. Read the glossary. You only have to declare force powers that give you an immediate attack. Lightsaber Defense, Block, do not give you an attack. Therefore, yes you can roll Djem So and see the result and then decide if you want to defense/block afterward.

Yeah, and to be honest, it's a very odd ruling. I've never understood why the timing of repost gets the special case, when it's almost irrelevant in terms of actual game effect, but the block/defense one is much more advantageous.

Here are the differences. With reposte and Djem so, the only difference is that you have to chose to spend a FP on reposte. In 95% of all cases, that's not a concern. It only matters in the minor case when your opponent will die from one return hit. Because in other cases, reposte is always the smarter ability, as it is 100% a free attack, whereas djem so is 50%. If you really need a kill, you just spend the fp on repost, and then if you miss, you still have djem so to fall back on. Kind of a strategic no brainer.

But in terms of block/defense, it's very advantageous 100% of the time to roll the djem so, then decide if you want to try to negate the attack or reroll the djem so. Again, kind of a no-brainer to roll the djem so, then decide if you want to reroll a fail, or roll a block/defense.

Anyways, it is what it is, and that's fine, just always found it a bit odd strategically and would have loved to asked Rob what the concern was that led them to consider reposte, but not block/defense, when clearly the block/defense is the more powerful combo of waiting than reposte.