Just updating this topic, alot of support is handled via e-mail as opposed to here on the Support Forums.
The required columns when merging are SKU, Product Name, Category and Price. When doing a standard import SKU is not required but is highly recommended in order to use the merge method.
Images are case sensitive, I’ve looked into adding case insensitive support but at this time there is no option to toggle this. I will plan to include this in the 3.1.9 Plugin update.
The 500 Internal Server Errors have been due to excessive memory requirements of this Plugin, loading and processing CSV’s chews alot of memory, the upcoming 3.1.8 Plugin update (due today or tomorrow) resolves most of these issues and displays a prompt where it cannot acquire sufficient memory allocation and import time limits.
Happy to discuss further here on support otherwise via our support e-mail account; firstname.lastname@example.org.