Let's pick a very Swedish situation. You've had some meatballs at Ikea, and now you're choosing between two kitchen tables (a table - ett bord; two tables - två bord).
The technically correct question would be vilket bord vill du ha? ("which table do you want?"), which shortens to vilket vill du ha? ("which do you want?").
But you'll find that if the ett-word is left out of the sentence, a lot of people will just say vilken vill du ha? as a kind of general default, because there's no remaining word in the sentence which needs to agree with the ett conjugation.
Hope that was clear enough. :)
-dera sure reminds me of DE deren and SV deras with "ingen" being normally used as EN "no" DE "kein" it sure seems quite a rather direct equivalent of "none of those/these" as in "none belonging to the aforementioned ones". In English you could say "neither of those/these" if you wanted to be more complete, which is equivalent in this specific case. Also, "neither" can also be "heller" if I'm not mistaken so a 1:1 mapping in all cases doesn't really seem to hold up anyway. But I kind of get what you mean.
The problem is that "neither" and ingendera are used for when there are two options, not more. When you have more than two, you use "none of them". Hence, it's not a great translation in practice, since it would imply that there are three or more when the Swedish implies just two.
"neither" can't be just heller, but it can be inte heller - literally "not either".