I was thinking about buying a Dacia Logan in Poland and bringing it back to Sweden . No problem, Renault in Sweden said that they can service my car BUT WILL NOT honor my Dacia/Renault warranty! Renault says they own the Dacia factory so why wont Renault honor my warranty in Sweden!

I don't think that all warranties necessarily apply to all countries - they may apply just to the country where the product is sold. Maybe this is the case for Dacia, it is the producer's choice to do that.
I sympathize with you, but I think that legally, Renault has the right to do this, and they probably do it in order to be able to control the prices in each country (i.e. to discourage buyers from buying from countries where Logan is cheaper and/or was introduced earlier).

If you can get the service of your car in SE you can still buy it from Poland considering the short distance and the ferry between Malmoe and Szczcin in case of....
But given the reliability proven by LGN so far you should not worry that much.
Nevertheless, before taking it to SE you should adapt the lights to Swedish regulations.

Just out of curiossity, do you have to do some kind of homologization (approval) before you can even register a new type of car in Sweeden?

That is neccecary in Croatia for car models (or model series) that have never been registered in Croatia. Sometimes it's not worth the trouble if there too much modifications to be made and sometimes it's only a formallity.