1) Is there a limit to the number of columns allowed in the gradebook?
2) Is there a character limit or technical issues related to the length of assignment names?
3) What is the character limit of an assignment name within the gradebook column?
There are limits on the lengths of names. The way to find out is to look at the definition of the corresponding database table. In mdl_assign and mdl_grade_item, what is the type of the name field? (It will be CHAR(100) or something, meaning 100 characters limit.)
There is typically no hard limit for thing like 'How many columns in a gradebook' or 'How many activities in a course'. However, as you get more and more of them, there is more data for Moodle to process, and things gradually get slower. Mostly, the number of things you have to add to make Moodle unusably slow is so large that this is not a practical worry.