You're throwing two different things together: onomatopoeia and actual interjections. The boy is not saying "gulp" (that would be funny), he is gulping. These two things have to be treated differently!
Onomatopoeia
In English, onomatopoeia is usually both a verb and a noun. "The boy gulped" and "The boy heard a gulp." In German, it's only a verb, and the noun is just the infinitive of the verb. "Der Junge schluckte.", "Der Junge hörte ein Schlucken."
How you express onomatopoeia depends heavily on the format. If you're writing prose, you're usually stuck to just using it as a verb: "Der Junge schluckte." with the added benefit of being able to describe it: "Der Junge musste laut schlucken" etc.
In comics, you can use the inflective (usually outside of the speech bubbles), which is just the verb without ending, like "schluck". This form is very limited, it can only be used on its own. It is informal, but this could be used in prose, too.
In dialog-based writing (like in a game) you can use the inflected verb in asterisks, like *hustet*.
Actual interjections
Disclaimer: Actual interjections exist outside the normal phonetical rules of a language and are therefore hard to write. Really hard. Furthermore, they are informal, so they differ by speaker. A lot.
Hmm /hm:/: Yes, this can be used as some kind of all-purpose question. In your case, I would probably just say mm /m:/, without a /h/ at the beginning. For me, this /h/ indicates deeper thinking, wondering about something etc. But for writing, I would stick to "Hmm" because "Mm" (however you write it) can mean a ton of other things based on intonation, from "leave me alone" to "I have no time" to an eager "Yes".
Whoa: I'd say a simple "Nein" is best here. The most canonical answer to a misunderstanding is always "nein".
Why not "langsam"? It has a bit of a reprimanding tone. If the beggar was significantly older than the valkyrie, or there were close, it would be fine. Else I would feel like the beggar is overstepping her bounds and being slightly rude.
Why not "boah" (pronunciation is like "Bor", not like the snake "Boa"). "Boah" is a surprise that is either very positive or negative, but never neutral. The beggar is obviously not super hyped, so "boah" would come out as annoyed.
English "whoa" would also be understood. The main problem with that is that the most sensible way to write this in German would be "wow", which overlaps with "wow" /wau/ which in turn can't be written "Wau" because this is already the sound of the dog /vau/. On the other hand, writing "whoa" would not always be understood: I had to google "whoa" because I always assumed it was pronounced /wa:/ (pleasant surprise, written "wah").