You can't add watermarks to photos in the catalog, this is not a feature of Lightroom. You can only add watermarks when you export the photo, and at that time there's really nothing stopping you from adding the watermark to every single photo that is exported (in fact, once you turn on the option to add a watermark at export, that option stays on until you turn it off). You can also add watermarks when you upload a photo or create a web page, etc.
1. It usually isn’t recommended to add a visible watermark to originals in a catalog, because then you would no longer have a clean un-watermarked version. That’s why Lightroom Classic is set up to watermark as many images you want on export, so that the copies have watermarks while the originals are undisturbed. But maybe you have a specific need, like a catalog that stores only non-raw images for online use? If you really need images to be watermarked within a catalog, you should reply with more detail about the specific workflow you’re trying to support.
2. You have several options for getting client feedback. In Lightroom Classic itself, you could create Collections, then sync them to Lightroom on the web, so that clients can see them in a web browser and make comments. But you may find it kind of limited. You can read about how to do that here:
Or, if you already have an account on a photo website that has features to help you manage client feedback and delivery (such as PhotoShelter, Smugmug, etc.), you can export from Lightroom Classic, upload those photos to your client management area on the website, and use their services to get client feedback. But even for services like those, you would typically not watermark what you upload. Why? Because if the client orders a photo, they need the final purchase to be non-watermarked. Some websites let you watermark what is displayed to the client for proofing, while leaving the original unchanged for the final print or purchase. Also, some of those websites have Lightroom Classic plug-ins to make exporting and managing a lot easier.