It can't even be called a slang. Someone wrote some characters together, and as a whole, its pronunciation is a little similar to the sentence "I love you". It's not a Chinese phrase, but it is not hard to know what it means if you know the pronunciation of the characters.
These irregular expressions make Chinese extremely difficult to understand to ...
I think what the others have said is correct, but I should also add that you can in fact use 请问 not just as a polite or softening start to the sentence.
As with the spoken language, everything is about context and the way you express it (something more difficult to convey in written language). So a teacher could say 请问你的作业在哪里? This can be used emphasize ...
It is very straightforward. 请问 simply means "excuse me" (as you yourself indicated in the example 'excuse me, where is the toilet?')
请问 has the same softening effect to the inquisitiveness of a question and it can be applied in similar social situations as "excuse me", "pardon me", "I would like to ask whether..." etc.
Yeah you've pretty much got it.
请问 is just a good, polite, way of giving the other person a heads up that you want to get some information out of them.
It's not necessarily demanding to drop the 请问, it all depends on your tone of voice.
If you're looking for more demanding way just drop the 在哪里 altogether and add a 呢 on the end while being overbearing. 呢 ...