Jean,
1. When I had my papers translated and legalized by the Dominican Consulate here in Canada they were only valid for 3 months.
2. The cost of your wedding will depend entirely on the type of wedding you are having. There used to be a price list on the Junto Central website
JCE.DO - Junta Central Electoral de la República Dominicana which listed the judge's fee (but I couldn't find it this morning). There are different fees for 2 Dominicans marrying, 1 Dominican and 1 foreigner and 2 foreigners. There are also different prices depending on if you go to the judge's office or if the judge goes offsite to your wedding location. Your fiancee should be able to get this information for you.
3. I'm not sure how EU countries work for issuing visas. Is there no local government website that can give you information? This is what we have in Canada.
4. Again, not sure about the process for your country, but yes in Canada there are cases where a spouse is denied a visa.