SimpleHTML don't support <table> elements, no <input>, no <select>, can only be used as some guide with hyper-links.
For the datagrid, I only suggest two things:
1. don't create one widget element row for each data row, just draw the rows that the panel can hold, and refresh them when the scroll bar's value is changed.
2. Try using editbox for each cell, you can control it by EnableMouse API, disable it to make it works like a fontstring, enable it for editing.
I created a DataGrid class long time ago, it's defined in a full oop language style, can't say it's in a simpler way because it's based on a big lib, and it's not finished yet because I don't see where I can use it.
It's abandoned also because for now I have an element panel interface, I can give any type of element (like button, fontString, also custom widget classes) to it with row count, column count, horizontal spacing, vertical spacing, margin settings and etc to generate a grid, the grid don't really care what type the elements are. So, if you want build some framework, you also can try to build something like this to do the dirty work.