"¡Por favor para!"
That doesn't sound right to me at all. 'Alto' as 'stop' is always a noun or an interjection. I guess you could say it, but if you insisted on saying it, this is one of the rare times that I would think it preferable to say 'por favor' at the end. I just wouldn't do it at all.
Somebody who has the authority to demand that you 'Halt!' is not likely to say 'please'. I've never heard anybody use them together, but I can't tell you it's grammatically incorrect. 'Parar' is a verb and can be used politely or shouted. It has a much wider range of usefulness. You can use it in almost every sense of the English verb 'stop'. The noun 'parada' can be used for most uses of the English noun 'stop'.
I've learnt that in Mexico and Central America, you see the word 'Alto' on road signs meaning 'Halt.' In my opinion, I wouldn't really think that you would add a 'please' if asking someone to 'halt'. It just seems more of a command than a request. So I wouldn't say it should be valid, even though the meanings are very similar. I'm not an expert though, so don't take my word for it. It may be perfectly fine to use :)
Well the imperative is by its very nature still a command. And "Por favor PARA" is in the imperative mood. It's a command, not a request, even if a polite one. A request is asking, not telling, e.g. "Would you stop, please?" or to make it even gentler "Would you mind stopping, please?"
"Por favor para", you are still telling/commanding the person to "stop", not asking.
"Alto" on the other hand is a noun, but in its common use e.g. on road signs it is also an INTERJECTION, which is its own part of speech. And which is indeed more curt & typically harsher than a command. It carries a connotation of URGENCY, immediacy.
An example in English turning noun into interjection, shouting "Silence!" (Actually in colloquial English people sometimes borrow Spanish "Silencio!")
Which is a much more curt & forceful way of way of saying "Be SILENT right NOW!". "Silence." as a noun cannot ordinarily be a complete sentence, but it can as an interjection meaning "Let there be silence."
Another example of an interjection which both English & Spanish share is "Caution!" "¡Cuidado!" that you can find on bright yellow signs. It's not a command form from a verb which would maybe be like "Use caution!" or "Be cautious!" in English. In Spanish using the imperative verb form of "cuidar" like "¡Cuídate!" or "¡Cuídese!" doesn't carry the sense of urgency, it would be more like saying "Take care of yourself!", not an urgent "Careful!", don't come any closer, there's high voltage power lines right here that will electrocute you.
There's no rule against adding "Please" or "Por favor" to those signs underneath "Caution!"/"¡Cuidado!", but it would kinda defeat the point of interjections and the curt one-word sense of urgency they convey.
But a teacher frustrated by a class full of noisy unruly students might scream "SILENCE!" Then take a deep breath and sigh out "Please!" to soften the harshness of her outburst.
In this same exercise para can be translated as stand. I stand here and you there. Yet here when I entered "Please stand," it was deemed to be incorrect. Any ideas as to why "Please stand" is not accepted while stand and stop are accepted as interchangeable in the sentence I stand/stop here and you there?
Close but not quite, "se para" is not imperative. One letter can change everything. "Se para" is the 3rd person present INDICATIVE form, you would just be saying factually "He/she/it stops/stands up." Or usted/formal "You stop/stand up."
The imperative form "para" is informal singular you/tú, so it takes "te" as object pronoun, "te para".
For formal singular you/usted imperative, you use the 3rd person present SUBJUNCTIVE form, so "se pare".
And for a reflexive imperative, the pronoun should be suffixed to the command, so for "pararse": "Por favor párese" or "Por favor párate".
"Stand up" as an alternate meaning for "pararse" is an Americanism, one could alternately use the less ambiguous "levántate" or "levántese" in the sense of telling someone who is seated/laying down to stand up.
Even though it has the same letters, the context would tell us that it is different. We have words in English also that have more than one meaning for the same combination of letters. Please for, didn't make much sense, but the conjugation of parar --> para did make sense.
I did not remember that; thanks. But I didn't drive while I was there, either. About that comma, think of punctuation as the highway signs of speech and reading. In an emergency, if a child is about to touch something that will hurt him, you would not use the polite "please" at all. If you have time to be polite, use both the "por favor" and the comma. :-)