Seeking help with referent of "der" in a sentence

[edmont]

Am Ostersonntag, dem 20. April 1919, kam Fritz mit seiner Frau Jane und Karl, der in den letzten Wochen wegen der Drucklegung des Buches zwischen der Schweiz und Württemberg hin- und hergependelt war, in Stuttgart an.


Does the highlighted "der" in the above sentence refer only to Karl? Could it refer to Karl and Jane? Or perhaps even to Fritz alone, since other context shows he is the main person in the story? 


Thank you for any assistance.  If possible, explanation with a rule is great, but intuition is great too.

[Keswick]

"Der" in this context refers to Karl. Fritz arrived with his wife Jane, and Karl, who due to the printing of the book....

I am afraid I can't give you a proper grammatical explanation but maybe tiorthan can, he's a grammar genius!

[Duckduck]

Am Ostersonntag, dem 20. April 1919, kam Fritz mit seiner Frau Jane und Karl, der in den letzten Wochen wegen der Drucklegung des Buches zwischen der Schweiz und Württemberg hin- und hergependelt war, in Stuttgart an.


Does the highlighted "der" in the above sentence refer only to Karl? Yes it does, as Keswick already said.  And I can give you one or two reasons if you like (and I know you do...).
1. As a rule the relative pronoun, introducing the relative clause,  follows the noun (or whatever is its referent) directly.
2. In this case the relative clause can't refer to Karl and Jane (and maybe even Fritz) because the relative pronoun (der) and the auxiliary at the end of the relative clause (war) would have to be in the plural.
Since your German is so good you might want to check this, even if it is in German:
http://mein-deutschbuch.de/relativprono ... infuehrung

Could it refer to Karl and Jane? Or perhaps even to Fritz alone, since other context shows he is the main person in the story? Nope


Thank you for any assistance.  If possible, explanation with a rule is great, but intuition is great too.

Cheers,
Duckduck
(who's not so bad in grammar either, dearest Keswick... )

[Keswick]

(who's not so bad in grammar either, dearest Keswick... )

Absolutely correct, but I hadn't seen you online in a while. Nowt bad intended, dearest! 

[edmont]

Keswick and Duckduck, thank you both!  So good to be able to hear from a community of experts who can confirm or question each other's conclusions. Duckduck, thank you for the crystalline explanation and the link!