Hello, I post rarely here but I feel like I can contribute to this topic as someone who speaks several languages on a regular basis (German being one of them, though not native).
First, generally adding accents rarely adds anything for me. It makes it harder to read while adding almost zero personality to the character. Basically the payoff is very small IMHO, especially because it often ends up being simple stereotyping. Especially since the character probably will be trying their hardest NOT to sound like a foreigner.
Secondly, in my experience (as in my class in high school) native German speakers who aren't terribly good at english do confuse the v and w sounds. The reason is that the German w (as in "Wald") sounds much more like the English v (as in "Verb"), meaning that the lower lip actually touches the teeth, while the German v sounds like an f ("Vogel" sounds like "Fogel"). However: They were people not interested in learning English and they did not feel comfortable speaking English at all. Those in my class who actually liked English and spent more time hearing it (such as choosing subs over the ever-present dubs here) didn't confuse those sounds at all. Basically: It's an elemental newbie mistake that a somewhat fluent speaker of English corrects because they CAN hear the difference with enough exposure. It's not an accent that happens because they can't pronounce the sound. TL; DR: Substituting V for W probably won't resonate with German speakers who know English and will only make the text harder to read for everyone.
Thirdly, on substituting words: This is a trickier topic because it can happen for two general reasons, habit or convenience. Habit would for example be the reflex "what?" in your native language. Probably won't happen with greetings, as I'll elaborate on further down in the post.
With convenience I mean the following: My family have three languages we all know (English, Swedish, German), and depending on the topic, the first and sometimes only word that I can remember spontaneously is in the language I use it the most in. Therefore, but ONLY when I speak to them (because I know that they understand it!) I'll use that word. I don't do this with people with whom I only have one language in common.
There's a third option that I'd advocate, because it does happen a lot, feels natural and is an excellent opportunity for some comedy, and that is doing what Ziva does in NCIS: Mistranslate and/or confuse words. Here, have some examples.
The other day my sister wanted to say "be ejected in a capsule into space" in Swedish. However, she thought of the English word "capsule", which lead her to use the phonetically similar "kapsyl" in Swedish instead of "kapsel". Meaning she said "ejected on a bottle cap into space" instead. Or I used to say "Gläser" (= Drinking glasses) instead of "Brillen" (= Glasses that help you see!) all the time in German because it sounds so similar to the English "glasses"! The basis here is phonetic similarity. This, as well as similarity in meaning, can also be the reason for confusing two English words when a foreign native speaks English. For a nice list of examples, look at this wonderful page of Ziva-Isms.
Now this would require knowing some German, or some sleuthing into mistakes that Germans might make in English. But it does feel more rewarding to me, as it's close to reality and gives the character some personality as opposed to following a formula someone came up with. In the case of NCIS, it's used to show the dynamics in the team, and especially DiNozzo gets so used to correcting her "mis-speaks" that it's purely automatic later on, and she's perfectly fine with it. (Shipping intensified)
As for the "Hola, everyone!" variant: I have never, ever heard this happen. Why? Because if someone is good enough at speaking English, they will know the most basic words like "hello" and "welcome" and NOT confuse them permanently! Yes, it might happen once or twice in the story out of habit (like they're caught by surprise), but let's be honest, even then it's rare. If someone suddenly shows up and says "hello!" to you, you instinctively say the same thing back. And if the native is the one approaching a group to greet them, they will be in "English mode" most likely and get it right from the beginning unless they're terribly nervous. Which can happen, of course! And I'm not saying that it doesn't happen, and you might do it on purpose like an inside joke with your friends! Only that confusing the simplest words is very rare and that it therefore feels more like a stereotype and lazy writing. I mean, if even I who don't speak Spanish know those words, they're simple enough to know in a language you actually speak.
All that being said, I have accidentally started speaking Swedish to my Swiss friends...