The two fonts you mention that "come with Lightroom" are probably intended for use by Lightroom for its displays, and not installed as system fonts. I wouldn't expect any fonts to come preinstalled with Lightroom for use by you, there's just no reason to provide fonts with it.

Also, unless you can find a document telling you you have the right to use that font for your own work, you wouldn't be allowed to use any such bundled font for your work (same copyright rules as for images: just you having an image from someone else does not allow you to publish it)

But there are many fonts available which you can legally download and use. Have a look at e.g. Google fonts.

About the "others to use them on various systems":

• if the "them" refers to your watermarked photos, you can use any font you want (and for which you have the proper licence), others will not be able to access the fonts through your watermarked images, as the few characters you have used are now pixels in the image and not a font.
• if the "them" refers to the fonts, you'll have to install those on the various systems according to the license of the font.