As one of those writers who has their fortune being seized - what canon doesn't support that? Canon ends with the war and then we get a brief glimpse of the Malfoys 19 years later. If you have some super secret info on what happened in those 19 years, please share.
As for their financial situation, yes, I think they are quite well-to-do. Some fics are insisting that the entire Malfoy family fortune was seized after the war, which appears to be completely wishful thinking. (It doesn't bother me so much when the writers under age 18 do it, but the writers over age 35 should know better!) Not only is there nothing in canon to support it,