Trying to figure out how to put all the events and timeline(s) together in a realistic way has developed into a very amusing discussion! I wish Robin had kept his view to himself though, because I like it when there is no obvious right or wrong interpretation...

And if you for a moment silence your wish for logic and reason there is a layer of a more poetic and emotional nature in this story to be discovered:

Old Thorgal feels that the world he knew is no longer.  (Because his beloved Aaricia is no more, this is a reality in which he doesn't want to be part)

He wishes he was able to go back. (The temptation would be irresistible!).

He is angry with this fate, speculating other choices or deeds could have made a difference. (He would even kill in cold blood, if it could bring her back!)

Deep down he realizes that whatever he tried it would only give him pain in the end. (Nothing would change, she would still be dead.)

Finally accepting what has passed, his life is at a new beginning; a journey on his own, to unknown destinies. (Like a young Thorgal ready to sail.)

Something gives him confidence that it will be all right - but different. (Her love will follow him where ever he goes)

Logic and reason back on:
In the end, everything could have been a dream of old Thorgal who fell asleep at that beach while watching the flames devour the drakkar. A dream combining many past memories and intertwining them with the different stages of sorrow. The story wouldn't be less real or honest if it indeed all was a dream, because some Thorgal albums take place in his own mind without obvious seams.

Of course Thorgal is attracted to Kriss' appearance. That doesn't mean he's attracted to her personality - on the contrary.

The first time Thorgal and Tjall sees her they say:
-Say that girl is superb!
-A wild cat's beauty Tjall, mind her claws.

Looks very very promising, I'm very enthusiastic!
Nice composition of dark silent boxes and explosive emotions.

I noticed a small similarity in the last box of the 'prelude' and the first page of Adieu Aaricia (album HERE )
There are two boats, in the first we see the mast break down from the fire, in the second there is a boat with a broken mast lying on the  shore. It's the kind of detail that doesn't have a meaning in the story per se, but for the reader it enhances the air of connection and time going in circles.

This picture from Alaska shows the frame of a traditional umiak, a boat also used by Inuits in Greenland. Its made of whale bone, probably because wood is hard to find in these areas. A bit mindboggling, the harsh conditions in which people actually live and lived. The arch reminds me of the album cover for Tupilaks.

The bright colours of Gaétan Georges picture differ a bit from the colours we are used to in Thorgal, which could be an additional reason to believe it is not depicting anything "real", but either a memory, a dream, something imagined... or a (false?) vision to lure Thorgal to surrender. The white bubbles suggest someone - who isn't in the scene -  is thinking.

I was also wondering where/when this would be. The mediterranean island in Arachnea doesn't fit with the spruce (épicéa) trees in the back, it has to be a more northern location.

Lovely drawing!
But he forgot the dog, didn't he...?

Thanks Alex, maybe that last post was a bit tainted with my own sadness.
With some more perspective, I'm realizing that what Thorgal would witness going back to the times he wasn't there would also be how strongly and bravely Aaricia always acted, refusing to give up even when (presumably) left alone in a harsh world. Yes, she might cry in her pillow at night and beg those wicked gods to bring him back... And he would be devastated to see that. But then she would also lift her blue eyes to the sky and put one foot before the other, knowing she - life - must go on. No matter what the circumstances.
And Thorgal would perhaps learn that neither can he allow himself to succumb to these feelings. Honoring Aaricias memory wouldn't be compatible with willingly fading away (like he almost did that once). In this mature age, he would understand that his responsibilities are not finished in the world, that he must live his life to its own end. 

In that last box, it would be Thorgal who lifted his eyes to the sky.
And meet the colour of hers.

Having lost a loved one recently, this theme resonates with me. 
Whether there is a Nidhoegg pushing for it or not, one cannot help but wander through past memories. Joys and sorrows coming back...
I wonder what scenes we will revisit from the life of Thorgal and Aaricia. Will he find out how much she struggled without him? What she told the children about their father when he was away for so long? Will he be able to stroke the tears of her cheeks as a prisoner of Kriss and Shaïgan?

I cannot see a happy ending here though, only bittersweet ones.

Thorgal is always fighting the physical barriers of this world - climbing rocks, swimming the ocean, falling through the abyss, trodding through deep snow or swamps, flying the skies, jumping, dodging, hanging... Given he got his name from Aegir, the God of the Sea it is no surprise we also find him on a boat or raft from time to time. He knows how to dive and in The Scarlet Fire we even see him forcibly submerged in a few boxes. But I still think there is room for more underwater scenarios  for our hero. Both his mother and the ancient Atlanteans drowned - or at least were lost at sea...
Who knows what lies hidden in the depths?

"The sunken mother"?

An underwater god?

Atlantean artifacts? (the  Antikythera mechanism comes to mind) 

Remains of a lost people... Or religiously sacrificed victims?

Maybe the Nine daughters of Aegir /Nine wave maidens?
(Modern illustration of Kolga, the "cold-wave maiden", below)