With over 400 chromosomes, does mitosis in a species of butterfly happen in the same way as in humans?
Mitosis was displayed in a lot of organism, also in organism with a very high number of chromosomes. Here's an example of the analysis made in a fish with an average count of 280 chromosomes. Look at the metaphasis of a cell with higher number of chromosomes (Acipenser transmontanus):



This doesn't mean that every chromosome is equal to others: there're microchromosomes, acrocentric chromosomes, meta, sub-meta.. So, the size required into the nucleus (because where're obviously talking about eukaryotic cells) isn't strictly connected with the chromosomes number. 

The answer to your question isn't a clear "yes" : the high number of chromosome in a butterfly is connected with a karyotype instability, where some cells received a wrong number of chromosomes into an alterated mitosis. Actually isn't fully clear why and how these errors come out. Here's something else about butterfly karyotype and phylogeny.