I am an Indian citizen with a Slovenia multiple entry visa.
Can I enter and stay in Croatia for major part of my itinerary?
This one is actually a tricky question.
As the website of Ministry of Foreign and European Affairs of Croatia states, you are allowed to enter Croatia with your multiple entry Schengen Visa:
Aliens who are holders of uniform visa (C) for two or multiple entries, valid for all Schengen Area Member States do not require a visa for transit or intended stays in the territory of Croatia not exceeding 90 days in any 180-day period.
This issue has been already resolved in the relevant question.
But your question is about whether you can stay in Croatia for major part of your itinerary. As Schengen Visa official FAQ states:
5. Where do I submit my visa application?
You must lodge the application for a Schengen visa at the Consulate of the country that you intend to visit, or – if you intend to visit more than one Schengen State, the Consulate of the country of your primary destination (i.e. main purpose of stay or longest stay).
But despite Croatia being the part of EU, it's not yet a Schengen state, which means the above doesn't apply to your case and you can stay in Croatia for major part of your itinerary, because your stay in Croatia is not considered as a stay in Schengen Area.
As Visit Croatia suggests:
This also means that if you are in possession of a Schengen visa, you will not use up any days of the time limit (90 days in a 180 day period) that you are allowed to be in the Schengen zone if you visit Croatia.
So it may come in handy, when you are travelling through Europe and need to be careful with the number of days you use up, you will still be travelling in Europe but won't be using your "Shengen Visa days". This procedure is maintained at the border: when you exit Schengen Area you'll get the exit stamp from Schengen and entry stamp for Croatia.