"Where is the bathroom?"
As a British English speaker this left me confused. Doesトイレ have the same meaning as bathroom in British English, i.e. the room with a bath, toilet and sink? Or does it literally mean the room with just a toilet?
In Japanese homes, the toilet is in a room separate from the bathing/shower facilities. 浴室（よくじょ）or 風呂場（ふろば）would be the names for these facilities. Often, the sink for hand-washing will be in yet another space, usually attached to or next to the shower/bath room. In this case, most people would just say 手を洗いたいけど… (I'd like to wash my hands...).
お手洗い （おてあらい）might literally mean '(a place to) wash hands', but in everyday Japanese, it could also refer to the place where bodily functions are performed.
[Side note: At temples and shrines, near entrances there are often places to wash one's hands and mouth with the ladles provided, purifying oneself before entering. These are often labeled as 御手洗, which is read as みたらい. In another context this could technically be read as おてあらい, but おてあらい is usually written with the first character being in hiragana, not kanji.]
Between BE, AE, and Japanese, there are enough dissimilarities to warrant a severing of any consideration of direct correlation.
No. Using が would place extra stress on the topic トイレ.
To illustrate: If someone were to say, 'The closet is there' and point at a door, which is in fact the door to the restroom, another person could say, 'No, the RESTROOM is there'. （トイレはそこです vs. トイレがそこです）
The fact that the 'subject' in the example is どこ, it would sound strange: 'WHERE is the bathroom?' ← too much emphasis.
It is best to use the 「ＡはＢです」 sequence for most expression that state 'A is B'.
Normally, Japanese places the verb at the end and the topic at the beginning. This is standard AはBです sequence. It could be rephrased to be more in line with English order, but it would be an aberration from the norm.
トイレ (topic) は (topic marker) どこ (question word, functions the same as an adjective would in its place) です (copula/verb) か (question indicator).
i got a problem with the translation of "bathroom". In a bathroom there is a bath. A bath is translated to ふろ. So my answer was ふろはどこですか。if i was to translate "トイレはどこですか" back to English i would say "where is the restroom?". Why? in america people have an opportunity to rest on a toilet. And Bathroom and Restroom are interchangeable.
The issue here is more with the indirectness (read ambiguity) of the US English terms indicating a place to urinate and/or defecate. Lavatory, wash room, ladies room, mens room, bathroom, and restroom all indicate the same thing: the place where I can 'do my business'/'relieve myself' (see, we can't even say pee or poo).
I hesitate to put a percentage on it, but most of the time someone will inquire about the location of the 'bathroom' it does not mean that they want to take a bath. In that case, they would probably ask 'Where is your shower/bath?' or 'Where can I shower/bathe?'.
I was raised in a home which included a room without bathing/shower facilities which we still referred to as a 'bathroom'. If anyone inquired as to the location of the bathroom we would point them to that room and had no complaints from anyone. So, your statement that 'In a bathroom there is a bath' is not universally true.
Finally, if you look at Japanese floor plans, they will indicate a 浴室 (よくしつ - bathing room). The room with a toilet, which is different, would be WC or トイレ. No ふろ or 風呂室/風呂場 is shown.
When います and あります are functioning as location identifier verbs, に must precede them.
The copula verb です functions like 'is' (A is B / A = B), but is not a used in the same sense as simple verbs. Grammatically, B is functioning in a non-location-specific manner, even if it is effectively describing a location.
トイレはどこですか？ The toilet is where?
トイレはどこにありますか？The toilet is located where?
ジョンさんは男ですか？ John is male? (A = B?)
ジョンさんは若いですか？John is young? (A = B?)
ジョンさんはどこですか？ John is where? (A = B?)
ジョンさんはどこにいますか？John is located where? (A = B location-specific)
I think that your confusion might be based more on a lack of understanding on how です is used as opposed to how あります is used.
You might want to read up on です specifically and understand how it is used as a catch-all predicate. Here is one source: https://8020japanese.com/desu/