I agree with what many others on here have said. It's also a personal thing. In general I like to try to force myself to learn only the minimum required to do what I need to do. If that philosophy sounds good to you, I would recommend taking the buggy version of frozen columns and try to fix the bugs. You may learn that the bugs are structural and you need to implement it differently, or you might be able to fix it with minimal changes. You will certainly get an understanding of the parts of slickgrid that you need to interact with to add this feature.

For the ajax data source thing, I would try to modify or extend the existing data source code to add the behavior you are looking for. As you mess around with it trying to figure out what you need to change, you will encounter the areas of the code that you need to understand.

With this sort of strategy you can avoid having to fully understand all the code while still being able to modify it. You might end up implementing stuff in a way which is not the best, but you will probably be able to implement it faster. It's the classic technical debt dilemma: understanding the complete codebase will allow you to design features that fit in better and are easier to maintain and enhance, but it will take a lot longer than just hacking something together that works.