@SureshR I don't know which post you are referring to, but if all the values are numeric, the sigma should show up. If the value is decimal, you can transform the datatype in "Edit Queries" and it will show the symbol.
Sticky answer to your second question. Yes, you can use parameters, but only in the Desktop and probably not in the way you describe. This video shows how Patrick Leblanc made this work off an Excel file, maybe you can see if there is a solution to your scenario.
It depends on your requirements. Edit table will only work in Power BI Desktop. If you need it to work on the end user side for just the collection of a few numbers then the PowerApps integration might help you. If you are looking for serious planning functionality, you might want to have a look at: www.acterys.com This solution offers sophisticated write back and planning for Power BI and Excel. In addition to writing to a relational table the solution offers a powerful OLAP in-memory engine on top of SQL Server that caters for large scale multi user write back requirements that typically don't work with sufficient performance if you just write to a relational table.
Wouldn’t it be nice if we could improve the user experience (ie. for those business users who haven’t a clue about PBI, “edit queries” or indeed sources within) to include a functionality whereby by the click of a button the input table would show up so they can edit away and then close and apply? This would make Power Bi much more powerful for “what if” scenario analyisis, rebates and discount finacial impact etc IMHO....