a) If you purchase classibase, you have access to all the themes/templates (i.e., Mukusic, Odsnocvet, Minimalist, etc.)
Yes, you will have access to all those themes and to future themes for time depending on your purchase. if you purchase yearly plan then you will have access for one year, if forever plan then you will have access forever.
b) can I use the script for multiple sites (domains)
Yes, you can use script for multiple domains.
c) The theme Minimalits, is from OSCLASS which is another script marketed by another company (osclass.org) .. or you are a kind of partners..?. and osclass is a free to use classified script. A bit confused here.
We are not partners with osclass. We plan to release premium osclass themes in future. Minimalist for osclass is free theme built to measure market interest. Future osclass themes will be paid and will look like existing Classibase themes. We did not start any work for those themes yet and there is no set date for them.