You seem to have missed the fact that you can enable field boundaries for fields--though in version 12, you will need to turn off the "delineate" option in Layout Setup... before you can see them in all rows of the layout.
But I'll warn you up front that the inclusion of sliding fields will create issues that make getting those borders to appear correctly difficult.
The trick, and it doesn't work perfectly, is to use the line tool to draw in vertical lines that start one pixel across the line into the footer (used to do this with the header, but the powers that be removed that option with recent versions) that then extends to just below the header/footer boundary line. You then add a horizontal line just below your row of sliding fields and set the horizontal line to slide up/resize enclosing part.
PS. It's best to post follow up questions/comments using post A Answer to add them to your original post. That set's up a thread of questions and responses where users can see the entire set of posts and you don't have to upload the same image all over again.
Also note that every application under the sun has it's own combination of things it does well and things it does not do as well. A databse is not a spreadsheet and a spreadsheet is not a database, thus there will be things that FileMaker can do that a spreadsheet app cannot do and vice versa.