Know about, plenty. Learn them…? None, unless you want to…? Learning to actually speak/use a conlang is something that goes beyond appreciating it. You need not learn a conlang to appreciate it. It’s an investment to learn any language, so you shouldn’t feel pressured to learn a language, created or otherwise, unless you want to.

As for languages I think more people should take a look at, I actually have a growing list, in that I’ve been giving an award to a conlang every year for the past twelve years. It’s called the Smiley Award. If you follow that link, I have a write up for all twelve of the languages that have won a Smiley Award, and links to their websites (or archives of their websites, if the websites are no longer on the web). The winners thus far have been Kalusa (group project initiated by Gary Shannon), Teonaht (Sally Caves), Ithkuil (John Quijada), Kēlen (Sylvia Sotomayor), ámman îar (David Bell), Okuna (Matt Pearson), Rikchik (Denis Moskowitz), Brithenig (Andrew Smith), Skerre (Doug Ball), Kash (Roger Mills), Ilaini (Irina Rempt), and Idrani (Trent Pehrson).

Neither. Or, rather, the term “natlang” or “conlang” wouldn’t be very informative in this case. For example, a creole like Tok Pisin is a natlang, but if you say that Tok Pisin is a natlang, English is a natlang, and Arabic is a natlang, I think an alien would be extremely puzzled by Tok Pisin. That’s because it’s most informative to say that Tok Pisin is a creole. Creole languages are special and have a lot of the same characteristics—characteristics which differ from other natlangs. Similarly, in this scenario, it’s vital to know that the resultant language evolved from a Western European auxlang—both to explain its vocabulary, and to explain its comparative simplicity.

This is a really bizarre scenario, though, because it would mean that the speakers would need to stop recognizing Esperanto as a conlang, and would need to stop having ready access to it. I just don’t see that happening. I also don’t see Esperanto speakers moving Esperanto away from Zamenhof’s in an unconscious, non-deliberate way. I suppose in a scenario where one Esperanto-speaking human was dropped off on an alien planet and refused to let them know that Esperanto was a conlang and didn’t flinch when the language was changed naturally over the course of time, it’d eventually become as complex as a regular natlang, but it would still be useful to know that the language(s) derived from an auxlang.

No, not fictional. I did create one to be used in real life, though I haven’t kept up with it. It was called KNSL, and was based on (i.e. kind of a signed variant of) my other to-be-used conlang Kelenala. Both of them are minimalist projects (fixed number of words). I also created a system for transcribing sign languages using ASCII (intended for use by conlangers created consign languages) called SLIPA, which you can read up on here.

Of course: Just create a language that doesn’t have them. Very simple. Whether a natural language does or not is tough. It’s an open question what counts as a diphthong in a natural language. I don’t think you’ll find a language where two vowels don’t come next to one another (maybe in the Caucasus), but it’s open to interpretation if a language will count two consecutive vowels as a diphthong. It’s a language-by-language thing, and often there will be differing interpretations. So…maybe? That’s the best I can say at the moment.

I don’t know about first language, but I’ve met three people who were raised speaking Esperanto and one other language, and they continue to speak it fluently. Two of them also spoke Russian as a first language; the other also spoke Korean. All of them now speak English (they now live in the US), but they mainly converse with each other in Esperanto.

And it has nothing to do with propaganda. It’s actually a practical solution. In the case of their parents, they met because they were Esperanto users and activists, each of their parents were from different places, and the only language they shared in common was Esperanto. Consequently, in the home, they spoke Esperanto. At school and in town they spoke the language of the country they lived in (Russian in Russia and Korean in South Korea). Esperantists often attend national and international gatherings, and, just like any con, people meet and fall in love. The difference is that rather than having a favorite show or movie in common, they have a language. If there is no other language in common, using Esperanto at home is a natural solution.

The major “do” for a conlang is have a specific goal. The worst conlangs are the ones that have no goal, because they end up not being good for anything, not even satisfying its creator. A major “don’t” for a conlang would be to copy any other language, either intentionally or unintentionally. Doing an a posteriori conlang doesn’t count, as that’s producing new material from an old source; that’s fine. Actually reproducing another language is not a good thing to do.

Also, while I recommending picking up as many natural languages as possible, if you’re interested in linguistics, I would like to say that linguistics and learning languages are separate things. There are plenty of linguistics who aren’t actually very good at learning languages, and aren’t as interested in it. My wife thisallegra looks at language learning as a chore, but she was a fine linguist. So don’t sweat it too much if you don’t have a knack for language but are interested in pursuing a degree in linguistics. It’s a big field, and there’s room for all types. :)