Mlem dev here. It shows strikethrough, but not superscript. Lemmy has got a unique dialect of Markdown - in the current version we’re using an open-source Github-dialect parser, which is close to Lemmy’s dialect but not perfect. In the next update we’ve written a custom parser that handles all of Lemmy’s Markdown elements, including superscript and spoilers.
Mlem dev here. You can attach a link to the post directly by clicking the link button in the post editor toolbar then tapping “Paste” (with a link copied to clipboard). We’re aware that this isn’t the most obvious UI, and are improving the design in the next version.