LR doesn't modify your original images. It stores the edits you have done in the catalog, in the file itself "As Edits" depending on the file or to a XMP file.
So if you have an image that was cropped when you Re-Develop that image and hit the Crop Tool the original image will be display with the original crop edit. you can remove that original crop and do another to a different aspect ratio or just resize the original crop to the new aspect ratio.
You can also Reset the image back to as it was when original imported. That is how LR works.
You could also create virtual copies in Lightroom, so previous edits/crops remain in place, and you are performing new edits on the virtual copy. In fact, I would recommend this approach.
To do this, you could select a photo, go to the develop module, then click on Reset to return the photo to its original unedited and uncropped state, press Ctrl-apostrophe (Cmd-apostrophe on a Mac) to make the virtual copy, return to the original edit and select the last edited state in the history to return that photo to its edited/cropped state.
Collections really have no application with regards to the issue of re-editing/re-cropping photos. Collections are useful for keeping certain photos together for some purpose.
I think using a collection would really simplify things when you are trying to isolate such a small number of images from a large library. Collections are relatively simple to understand, in my opinion. All you have to do is create a new collection, naming it anything you want. When you add images to a collection, you are not creating another copy. You are simply creating a link to the same image. And you can group images together in that collection from different folders and even different hard drives. So the collection would contain the 100-150 photos that you are talking about. Then you could just work on those images to crop them and prepare them for display on the TV. You could even create virtual copies and then filter to only show virtual copies and do all of your cropping and adjustments to those images. Then you can export those virtual copies as JPEG images which you can use to prepare your slideshow.
Obviously, there are a number of different ways to do things. I am only providing a couple of suggestions which you may or may not want to use.
Whenever I am creating a wedding album I will create a collection first and choose all of the images that I want in the album and put them into the collection. Then I crop and work on them until I think they are ready for the album. I export JPEG files from the collection and use those exported images to create my album. I don't use the book module in Lightroom. I prefer another software. But using the collections has really helped expedite my projects.
I think using a collection would really simplify things when you are trying to isolate such a small number of images from a large library.
Well, the original poster said that these photos would be isolated using flags, that seems sufficient to me to get the job done in this case. If you add picks flags to the desired photos, you don't really need to then further isolate these photos by putting them in a collection.
I apologize for the suggestion. So should I withdraw my post?
No, I just want the original poster to understand that she picked out a method that works for her selection of the desired photos.
And I was just trying to explain collections because the OP indicated that they had never used collections. Different methods, same results. I guess it's all a matter what works best for the individual.