Thanks zebec!

Okay, decision made.

We've booked four nights in Ljubljana, four nights in Bohinj Bistrica and four nights in Kobarid.

Thanks for all the suggestions!

......and as the bus from Kobarid to Cividale del Friuli runs only on Saturdays until mid September, you will have to backtrack to Most na Soci and to cross the Itallian border at Gorizia.
You can then reach the Dolomites by bus 110 from Tolmezzo (Tolmezzo dp 13.15 - Pieve di Cadore af 15.32 or by bus 440 (Tolmezzo dp 8.55 - Sto Stefano di Cadore ar 10.38).
Gorizia - Carnia by train. Carnia - Tolmezzo by bus.

After Kobarid we'll head to Villach, so may have to head back to Ljubljana, then take a train to Villach.

It's not forbidden to make the huge detour via Ljubljana.
Otherwise take the train from Most na Soca (Isonzo Bridge) to Jesenice and change there to a Villach bound train.
For example Most na Soci dp 8.12 - Bohinska Bistrica 8.52 - Lake Bed 9.12 - Jesenice ar 9.28, dp 10.47 - Villach ar 11.22.
Which places do you plan to visit in Carinthia?.

Yeah, we'll probably do whatever is fastest, haven't decided yet, still researching options.

As for Carinthia - Peč mountain, Dobratsch Nature Park, Lake Faak, Lake Ossiach.

Do you have some other suggestions?

And here still the timetable for the Dolomites leg:

Bozen dp 8.07 - line 920 - Vigo di Fassa ar 9.20:
Vigo di Fassa dp 9.57 . line B 101 - Canazei ar 10.20

Canazei dp 9.37 - line B 101 - Predazzo ar 10.29, dp 11.10 - line B 122 - Feltre ar 13.37, dp 14.20 -line 13 - Bellluno ar 15.07, dp 15.50 - ligne 2/3 - Alleghe ar 17.00

or in a much more easy manner:
Canazei dp 7.37 - Moena ar 8.15 - TAXI - Falcade dp 8.57 - Cencenighe ar 9.17, dp 10.15 - Alleghe ar 10.32

The (almost) final itinerary:

LJU
Bohinj
Kobarid
Villach
San Candido
San Vigilio di Marrebbe
Val Gardena
Merano
Treiste
LJU

For Trieste, our friends who live there suggested a few restaurants. We've eaten at the ones with an asterisk (all great meals).

Near the canal:
Le Bollicine*
Le Delizie*

Near the marina:
Trattoria Nero di Seppia
Antica Ghiacceretta*

Further Afield
Trattoria Suban
Tendra Rossa* (gorgeous views over the water)
Casa Pepe e Cucina (Neopolitan style pizza)

If you can fit some time in for Muggia, you won't be disappointed. It's a charming little village; and Trattoria La Marina serves a fabulous Fritto Misto.

Excellent, thanks fourfortravel.