The night is darkest just before dawn. Yesterday when I wrote the post above I was resigning to the idea I wouldn't get past chapter 2, but then with a bit of luck, I came upon a strategy that worked for me. It's a bit cumbersome, but I consider it an honourable attack strategy for this hopeless-looking situation, not cheesy in any way.

My party consists of the PC : Ranger 5/Rogue-assassin 3, Shadowheart, Lae'zel and Wyll, all 3 of them lvl 8.

Every attack with Jaheira ended in disaster, so obviously some kind of other route was needed.

Scouting around the tower I found a door, it leads to a number of empty rooms, but nothing that could help to gain some advantage in the central aisle where the combat takes place. There's a staircase down, but when you take it, are immediately attacked. It's a tough fight, but winnable. No one fell in my party, but we were low on hitpoints and spells. Another combat would be fatal, so a long rest was needed.

I decided to sneak out the same way I sneaked in and go to a safe place to travel to camp.
Now here's where the desperation set in. During my scouting, the harpers apparently had launched an attack and were obliterated. But as a result of this, the tower's troops had advanced up to the entry gate and so my group could no longer emerge unseen. We were immediately attacked and destroyed. All hope seemed gone.

So with this, I decided to go back to a save inside the cellar and look around. It was empty, so fortunately no new combat ensued, but there was another door that led to a dock-area. It was guarded, but when the guard spotted us, they saw we were true souls and let us pass. From the dock area we could get to a bridge leading to the shadow cursed battlefied area and we were safe.

Because of the guard's reaction, I thought that if we returned without a Harper's army on our tail, maybe the cultists would let us in as true souls, so after long rest we went back to the waypoint and entered the courtyard. Unfortunately we were attacked again before reaching the gate. So that plan was no good

Now however the battle was outside and the cultists' tactical advantage they had inside the tower was gone. It was a headon fight. They still had advantage in men and firepower, but were themselves more concentrated. Moreover my party could now retreat more easily and try to flee battle when things went wrong.

And that's how it turned out. I could take out 4 or 5 bozo's and then flee. (Only Lae'zel didn't make it in the first raid and had to be brought back by Withers)
And that's when I decided to try a hit-and-run tactic. Try to take out the strongest enemies in the courtyard, and flee when we were close to collapse. A total of three raids was sufficient to clear the courtyard.
This left the warden, 2 adepts and a few minor fighters within the central hall. And this was a "fair" fight which we won without any losses on my side.

So the strategy is :
Sneak in from the side, let the harpers go on a suicide mission, so the cultists are lured to the entrance of the tower
Sneak out via the docks and return to the waypoint to have the cultists attack you outside the tower.
Employ hit and run tactic to deplete their troops and recover yours.