Planning? Never...
Okay, I'm going to present a case for why we need to go to Caer Albion, ASAP for the purposes of the rebellion. The reasons are relatively simple:
We show the flag, as it were. A quick response shows that we're plugged in and we know what's going on around the continent and are interested in it, beyond our little corner and furthermore, are willing and able to respond. The presence of two spiritblades is an added bonus, but by no means should be our only or necessarily primary objective (in terms of what will have the maximum positive affect for the rebellion; as I understand it, the blades are ultimately a small, though important aspect of it. It's also likely that it comes down to some sort of Tain/Bluestar Spiritblade showdown once all of them have been collected by the various sides).
If we go after the blades on the Isle of Mourning, we remain (or appear as) a local factor, something that Tain can contain in the Oceanus region (or Prydein, which is the name of the island, IIRC) and therefore no real threat to the draconids and not much use to anyone else interested in standing up to the Forces of Darkness tm.
Appearing in and having an effect on events in Tain's capital city also shows that we are not powerless. Whatever we do (given some element of success, of course) can be seen as weakness on our opponent's part, as he cannot even keep a pesky band of rebels from acting in his own capital. Whether we get one or both spiritblades or not, a visible action by us doing something (defending the city, killing bad guys, evacuating people, whatever) will be more important than the substance. Remember, we don't have to win a military action to achieve success. Recall the North Vietnamese Tet Offensive...a complete military defeat, but ultimately an important step on their road to victory.
As for the meeting with the Janstenites...well, we have a rebellion to run and that needs to be our first priority, especially with events moving as they have. We have an opportunity in Caer Albion and missing that boat could hurt us more than any negative effects on negotiations with that faction. Indeed, success in our Caer Albion endeavor might put us on better ground for negotiations.
Anyway, just a few thoughts.