Using "ni" or "er" as singular is not recommended. It's normally not used in Sweden and is an old way of speaking that can be perceived as confusing for the person och cause missunderstandings. It is an old way of being polite but using it now can be be perceived as you are making fun of that person - kind of implying that the person thinks he/she/hen is better or more worth than you or others in some way, like higher class, and you looking down on that person beacuse of it. So and old way of being polite can now be used for being rude.
I have been browsing through the Swedish pronoun courses and get really surprised that nobody has ever explained the fundamentals of them. 1. The pronunciation of "de". Today the wast majority pronounce it "dom" the others pronounce it "di" (compare it with ni). Di was the only correct pronunciation for hundreds of years. "dom" was considered quite vulgar and was only used in northern Sweden. However, in Stockholm people started to use "dom" so now it is the accepted pronunciation. 1970-1980 "di" was still very common, especially in the south. Note, it has never been pronounced as "de"
Dem. Here you hear dom/dem. In Finland only dem, I think. In writing? Always use de/dem. Personally I find it hard to read texts with "dom".
Ni/ni - Er/er - The polite singular pronoun is always capitalized. It is getting more and custom to use the polite form again. It has been more or less dead for some decades but now it is rather common.