2. Is it true that I cannot "rename" a thumbnail which has been "developed"
There's one aspect of Lightroom's file renaming that often confuses photographers that I've worked with and which might lie behind your question. You can rename an original file from anything that's considered a master (or original) but not using a virtual copy. It's not a bug or defect but virtual copies are often misunderstood.
Let's say you have four renderings or versions of a single file sat next to each other in your library, one of which might be considered the master version ('original') and the other three virtual copies. Lightroom still isn't modifying any of your originals so really, all these renderings are peers, whether nominally labelled masters or not but because there's only one original image file, only one of these versions be linked directly to it and only that version offers you the option of renaming the original file. One approach is to nominate any of the virtual copies to
be the master; once done, you can provide a new file name using the usual mechanism. Open the Photo menu and choose Set as Master. This approach is suitable if you intend to export/print just one version.
need to append notations as a Caption in the metadata
That's actually the better approach and is particularly useful if any or all the virtual copies might be kept and exported for print at any time. It might be better to use the 'Copy name' field rather than Caption, though — it keeps Caption free for its correct use if you ever submit to a picture library or a publication.
Here's one way of doing it. In Library mode, choose to view 'All Plug-in Metadata' from the pop-up menu right next to the word Metadata in the right-hand panel. That will reveal the field labelled 'Copy name'. You can use whatever description you like in here for each copy — something simple like a, b and c or something more descriptive. For the smoothest workflow, you should also set up a filename preset (you only need to do this once) that includes 'copy name' in the filename when you export your TIFF. To do that, hit Export, go to the File Naming section, choose 'Edit…' from the pop-up menu and make use of the Copy name field that appears in the Image Name section. (You can combine it with other fields to automatically create useful names)
I can, if desired, import the exported TIFF itself back into LR so that it appears in the catalog for easy reference.
You can even do it while exporting. After hitting Export, tick the box that says 'Add to catalog' (it's in the Location section) and you won't need to re-import the TIFF. Lightroom will place it next to the original in your Lightroom library view, assuming you're sorting by something like capture time. (You can later choose to stack it with the other versions.)
When you've set up your export with the right format, bit depth, colour space, location and file-naming options and the 'add to catalog' flag, make an export preset of it (click Add when you've set all the options in the Export window, then provide a descriptive name) and the whole thing will be accomplished in a single click after that. Just remember to fill in the Copy name fields first.
