Let's start from the assumption that the only correct answer is "It depends" 😁

It could take 3 days if everything goes smoothly. There are rules that you just can't bend with Shopify / liquid. For example, pagination can be a nightmare. But if the translation to liquid is easy, a week seems reasonable.

I have never worked on a Swiss shop, so I can't be certain but there are paid apps that do those things. (Configure the VAT for Europe is doable help.shopify.com/en/manual/ta...)
Anyway every shop is different so it's possible that doesn't fit your needs

I have been working with Shopify for 4 years now. It's the best ecommerce solution out there. It's not perfect, and you may have to compromise, but usually is for the better. If you need any help feel free to get in touch so I can repay all those years of free JooQ 😁