I don't think I can offer very solid advice since so much depends on your preferences and interests and travel pace. I can tell you how much time I spent in some of these wonderful places, with the preface that I pack a lot in to my trips. Too, I was already an experienced traveler when I visited the places on your list, and so had learned a LOT about how to travel in a way that suited me. And I wasn't looking for either nightlife or beaches. With that in mind, I spent:

Kakrow- 2 Days - Given changes since I was there, I'd give it at least 3 days now.
High Tatras- 2 Days - I haven't been there yet.
Budapest- 3 Days - I had 3.5 very long, full (wonderful) days
Belgrade- 3 Days - I haven't been there yet.
Sarajevo- 2 Days - I had 1.5 and would have liked more
Mostar- 1 Day - 1 day worked for me.
Kotor- 3 Days - I visited Perast and Kotor as a long day-trip from Dubrovnik.
Dubrovnik- 3 Days - That's what I had in Dubrovnik, including the day trip to Perast & Kotor; the time worked for my purposes.
Split- 3 Days - I spent 1 day in Split (wish I'd had 2) and another 1/2 day or so in Trogir
Ilovik- - I haven't been there yet - sounds like a great option for you
Slovenia- 3 Days - I had 4 days and wish I had more
Venice- 2 Days - I had about 3.5 days and wish I had had more.

Again, it really depends on your interests and tastes.

You're idea of skipping Venice for this trip and re-routing has a lot of merits. Venice is wonderful, but you are trying to pack a lot in.

Hope that helps!

BTW, it looks like you flagged your post for Croatia only. You might contact the Forum's editor and ask to add flags for the other countries you are thinking of visiting. That might get you more responses.