How old are your relatives?
Never heard of required etiquette for using a spoon with spaghetti. Seen people elsewhere use it, others not. My father in-law got dirty looks and grumbles towards him once for cutting his spaghetti in Italy. I mean, Who cares!
Tastes the same, has the same texture and after a day it all looks the same....
Tipping soup bowls? Dunno, but I do know that an aforementioned Chinese way is a no-no, but I do it if it's just me and my wife. Hell, she does it to and she grew up here.
Wiping the plate until clean, only if it was super tasty sauce!
Manners are 'supposed' to be a learned behavior right? Tell them that's how you were raised, presumably by your parents/family.
My in-laws (both 70+) are super prudish when it comes to manners and stuff. Leave the room for less than 5 minutes while studying to assist my kid, come back and the lights are turned off. Chair not perfectly in place after getting up, ruffles feathers. Empty the dishwasher and not sub-organize the forks together into their matching sets! BAH. My wife and I had a blow-out on this stuff with them. Said it was not intentional and I got a "It's not your fault you were raised that way" response. LOL We got out of there lickity split less than 2 days later. Both my wife and I couldn't stand it. For the best, now they are all cool. Better as friends and not roomies.
Back to the spaghetti...
A quick Google search revealed how to eat it in Italy...
http://www.annamariavolpi.com/how_to_eat_spaghetti.html
Keep hands on table
NEVER USE A SPOON
Never cut the pasta
Don't slurp
Don't splatter
If they give you flack, say you did your research. Sure you are in Switzerland, but it's an Italian dish to their rules apply! Never use a spoon!
No to mention, if my kid is actually eating her dinner at her age (4) without putting up a fight, screw the manners! She's being quiet and eating. Manners will be taught later.