The default setting for import at once is 100 and my guess is, there's a good reason for that. Why did you change it to 100000? I can't see any point in that except that you are causing timeout and 500 errors. And why set the memory limit to 256MB? Maybe the value is too low? I am no expert, but I imported 200k list with the default settings without any hitch.
Why use these numbers? Speed the import? What's the point?
Try going back to default, like 100 or 200 at once. This will load a batch of 100, process them and go on to the next 100.
Server load will be way too high if you try to load 100k subs at once into a batch for processing. IMO