Split or Istria Region (Pula,Rovinj)

I was just in Pula and Rijeka ( Fiume in Italian). I cant imagine a safer place, and definitely not in most of America.

I agree about Split , as a "home base" it is too far from other parts of Croatia let alone other parts of Europe.

I speak Italian and I can say that in Pula one can get buy fairly easy in Italian, though most young people will speak English ( they learn from second grade). Older people depends on level of education, those who have gone to university will generally speak a very good level of English.  Funny though in both Pula and Rijeka some younger people will give you blank stares for any Italian past a sentence or two, and ask to speak English.

Rijeka is a larger city than Pula, but has advantage of train connections which Pula doesn't have, I don't remember about Rovinj maybe it has a connection to Trieste. Will you be driving or going by train ? Something to consider in choosing a home base. Also, looking where UK discount airlines fly you may wish to consider- often dirt cheap to fly and connect through London rather than flying from Croatia to a third destination. For example I could fly from Pula via London to Rome, cheaper than by train form Pula or flying to Rome from Pula.