What does the different seats do

This is a general answer rather than being specific to Disgaea.

From my experience of other game "series" you can play a sequel without having played the original. The first level(s) are usually easier and introduce you to the characters and their abilities - after all the writers can't assume that you've played the original. All games seem to have these "tutorial" levels.

What you might find is that, if you had played the original, you pick up the moves etc. more quickly, thus allowing you to complete the early level(s) more rapidly and get onto the meatier levels sooner.

Having a knowledge of the characters might make it easier to identify enemies and their attacks - but again you'll soon pick this up as you play.

I've experienced this issue a few times. The only way to fix it is to hold the power button down for 10-15 seconds until the unit turns off and and reboots. From the recovery menu select option 1 (reboot) to load the system as normal. It only takes 30 seconds to fix so not a big issue.

Unfortunately, I haven't identified the cause of the issue. It happens randomly, probably when it tries to connect to the internet (hence flashing blue lights) and crashes.