Tack / Snälla
I understand that both 'tack' and 'snälla' could be used to mean 'please' when making a request.
What I am wondering is, are there situations where only one word would work or are they interchangeable? For example, would Kan jag ha mer kaffe, tack?' and 'Kan jag ha mer kaffe, snälla?' both be acceptable?
In some situations they are interchangeable, but there's very different meaning and weight to them. I apologise in advance as this will get a little messy.
Snälla is a lot more pleading. Snäll means kind, so you're literally appealing to someone's kindness.
Tack means thank you, so you're thanking the person in advance. This is more formal (or I should really say general, as there's no real formal Swedish used in conversation) and, generally speaking, sounds more grown up.
So, in your example, using snälla makes it sound like you're pleading with the person to get more coffee. It might work in some circumstances when you're familiar with the person, but generally I'd stick to tack in that situation.
Snälla is more commonly used in situations where you want someone to take pity on you (though not exclusively), such as when a child asks for something, or when you want something not to happen.
Snälla pappa kan jag få en häst? Snälla! - Please dad can I have a horse? Please!
Snälla mamma tvinga mig inte att gå till skolan! - Please mom, don't make me go to school!
I've always used tack when I've been over there, it's nice to know it's the more common used phrase at least haha!
Also, the is the slightly formal Kan du vara snäll att... ? lit. "Could you be so kind as to... ?" that you can use to ask someone to do something. In that context, there is not the same sense of pleading and begging.
And of course the variant "Vill du vara snäll och...?" which translates to "Would you like to be kind and....?" (lit. Want you be kind and...?).
Having thought a bit more about these things after coming in contact with more people learning Swedish, in real life and here on Duolingo, it's been fun to realise there are so many little variants, edge cases and playful ways we mix up these kinds of phrases that I'd never even thought about before.
For anyone reading thinking this is starting to sound very complicated though, I'd say "Tack", "Tack så mycket" and "Varsågod" will get you through pretty much any situation where you'd want to say please, thank you, you're welcome or be my guest.