Memory works the same way for programming languages as it does for everything else: If you don't use it, you'll start forgetting it. If you start forgetting a language that you use then this might be a problem.

There was never a problem of filtering out the comments, the problem is that some programmers tend to rely on comments to explain their code instead of meaningful names and patterns. Meaningful names and patterns help even if you work alone or in a team. One scenario when comments make sense is when you have to provide a black-box library and need to provide information regarding you method usage.

It depends, the experience gained from challenges that you face working on side-projects might help you at work at some point. I was frequently in the scenario when the courses I learned first helped me in my side-project and only later at work. So I benefited from the experience from the side-project. Learning is the main part of the project, yes, maybe for the start, but after a while you don't learn that much during a project. In companies projects extend over a big period of time, and freelancing usually consists of smaller projects. Most programmers will do the same type of coding in their side-projects, web programmers will do web projects and so on, and will tend to use newer technologies. It takes a while for a company to adopt newer tech, so by allowing employees to do side-work will be a big benefit for them. Of course I'm not talking here about an abusive employee, I'm talking here about a responsible one :).

If you, as an employee, work on some side-projects during work hours and also manage to do your work-related stuff then this is, in my opinion, just personal and professional development, it's like doing a course on pluralsight, it helps you by offering you new challenges. On the other side, if you drop work-stuff in order to work on side-stuff then this might end up as a problem. So, I think that yes, a company should allow its developers to work on what they want as long as their work-related progress is on track. e.g.: pluralsight, courses, side-projects, open-source.

I've used C# to work with excel table. A good option was to use the Microsoft.Office.Interop.Excel library, this works very well, but it requires you to have Microsoft Excel installed. If you don't want to be dependent on Excel, then you can use NPOI library.