Imo, how clients are rendering the articles are a more pain in the ass rather than to publish one.
Discussion
Yes, I'm not overly happy with that either. I wrote my own article rendering for public HTML pages on our relay and there were quite a few edge cases from inconsistent usage to work out, too. Some clients repeat the title in the content for example. Lots of small issues like that. :/
Standardizing it in a NIP is good, but doesn't help with people using it wrong.
It's funny (and sad) that both of the issues in the original post there just stem from using Markdown wrong, and have nothing to do with Nostr or interop. Newlines without double spaces mean nothing in Markdown (for good reason, so you can format your source document better), and the image syntax is just plain wrong.
Clients should obviously at least implement Markdown rendering according to the Markdown spec, if they're not using a library that does it for them.
Parsing Markdown to HTML seems to be quite a challenge.
For my work / project I'm using https://commonmark.thephpleague.com/ but I can imagine the behaviour is different with other libraries
For the moment this is a challenge unrelated to Nostr (as a protocol) or NIP-23.