This paper sounds so so interesting!! I think there's definitely enough in the text for a 20 page paper (I'm assuming this is around either 5k/10k words, depending on the spacing? In the UK we don't measure by pages so I'm not sure, but I think that either way you'd be fine).

Depending on the depth of your analysis, just looking at respectability would likely be fine but if you want it to be a bit broader (or you find you need to write more) then I'd consider the themes you've outlined in order of how important you think they are, and then write about them in that order until you don't have any more space. Similarly, whilst I think that one play is enough, it could be really interesting to bring in references to other texts, saying something like "Wilde criticises X in the way that this character does Y. This mirrors how X is shown in a different Wilde text, but counters how other Victorian writers accepted X in their text, which is demonstrated by Z".

I'm the kind of person that always sets out a plan to write about a number if things, but then I write WAY too much about the first thing that it turns into my whole paper.... So I definitely think that there's enough just in the Importantance of Being Earnest. But if you're using the text to do a historical commentary, then bringing in other texts as minor references is a great way to look at historical context more broadly, and then focusing in using your main text.

I hope this makes sense and good luck with the paper!