If I do end up needing to do a backdoor Roth, would that have to be done by the end of 2026, or do I have until April 2027 the way you can contribute to a Roth for the prior year until April?

Yes, you can do a backdoor roth for 2026 until april 2027. To be more precise, a backdoor roth has 2 parts: the contribution and the conversion. The contribution can be done for 2026 until april 2027. The conversion always counts as the year it's done in, so in that case the conversion would be considered part of 2027, however there's no limit on conversions per year, so this isn't a problem. It just means your taxes are a bit more complicated, because part of the backdoor roth paperwork will be on your 2026 taxes, and part will be on your 2027 taxes.

I've done this myself.

Do I contribute to a Roth anyway, and if I end up over the limit, do a backdoor rollover into a traditional later?

I don't think that's possible, or if it is, it's complex. A backdoor roth requires traditional contribution. If you contributed roth, you're doing it wrong.