Surely that would be bafu iko wapi?
In many places the toilet and the bathroom are not necessarily the same room, and not everywhere uses bathroom euphemistically to mean toilet. It's far better to learn the two concepts separately, just to keep that awareness in mind.
It should but toilet is the term that is used more over bathroom. Bathroom is more formal for Europe/USA region.
There are many interesting ways to say this without mentioning the word toilet. Bathroom, men's room, ladies' and so on. I've also heard washroom being used (interestingly in Kenya or by Kenyans). I don't think Duo should accept all these euphemisms, but it would be interesting to know if there are any common ones in Swahili.
Choo often is omitted altogether. The one I know most/learned for it: "Wapi ninaweza kujisaidia?" - Where can I help myself?
My Tanzanian teacher just taught us this phrase and said we shouldn't actually mention "choo". So it seems they also prefer a euphemism.
Yes, 'ninaweza kujisaidia wapi?' is the most common term I have heard... Toilet is not strictly correct: it seems to be a translation from the French - toilette - which actually means to wash oneself, to perform personal hygiene - which is hardly the same as using the lavatory!