While most of what you say would work out fine if your wife was running a business as opposed to supplying services.

When it comes to supplying services there are a lot more rules and regulations involved because the government wants to ensure that the customer is not just using this arrangement to avoid their social responsibilities.

For instance the AHV will not accept your registration unless you can show them that you are running a real business with respect to business risks, multiple concurrent customers etc... and without this registration her potential customers would be held responsible for AHV payments and pension contributions.

For this reason most companies will not take on software engineers directly as contractors unless they come via some kind of agency or have a limited company of their own.

The best idea would be to operate via an agency or one of these invoicing firms that are setup to handle the red tape involved and to give comfort to the client that they are not going to be exposed to additional payments for AHV or similar charges.

Good luck with that,

Jim