We don't recommend deleting LingQs, since it's actually quite common to forget words you know. Having these LingQs still appear in your text is good review, and you also continue to benefit from the time you spent creating those LingQs. As you've noticed, deleting LingQs also causes them to be removed from your Known Words total. You can manually highlight them again in the text and mark them as Known, but by removing the LingQ, we assume that you don't want the word anymore.
I notice that you are new to the site and seem to be enjoying it, so I encourage you to consider upgrading just to check things out. It's only $10 a month and you can downgrade at any time, so if you don't like it then not much is lost. However, upgrading will give you access to some great features that we have on the site and should help you to learn your languages even more effectively.