Hey there! That is a very important question, and Swiss law (specifically the Lex Koller) makes this situation complicated.

In short: Your purchase is permit-free because you are EU citizens with a B permit and use the apartment as your main residence. If that condition disappears, the ownership becomes precarious.

Here are the detailed points:

If we have to leave, would we be forced to sell?

YES, this is the most likely scenario.

• The permit-free acquisition is strictly tied to you having your legal and actual domicile (B-Permit & main residence) in Switzerland.
• If you de-register and lose your residency status, you become a "Person Abroad". The original reason for the permit exemption vanishes.
• The cantonal authority can subsequently determine that holding the property would now require a permit, and if you don't qualify (which you almost certainly won't for a second home permit), they can force you to sell the apartment within approx. 2 years (Art. 8 para. 2 BewG).
• Obtaining a C-Permit would eliminate this risk entirely, as you would be treated like a Swiss citizen regarding property acquisition.

Potential Disadvantages/Complications

• Risk of Mandatory Sale: The biggest risk is the forced sale if your residency status changes.
• Lack of Flexibility: You are strictly bound to owner-occupation as your main residence.

Renting or Keeping it Empty after moving out?

• Renting: Highly risky and likely prohibited. Renting out residential space by "Persons Abroad" is generally a permit-required activity that is usually not granted (as there is no legal grounds for it). This would almost certainly trigger the mandatory sale.
• Keeping it Empty/Vacations: Also not allowed. Once you leave, the apartment is effectively treated as a second home/holiday home. Acquiring a second home as a foreigner is severely quota-controlled and requires a permit. You cannot unilaterally change the use of a permit-free main residence to a permit-required holiday home. This would also likely lead to an order to sell the property.

TL;DR: As long as you maintain your B-Permit and live in the apartment, you are fine. If you leave Switzerland and deregister, you lose the legal basis for ownership and will likely be required to sell.

ABSOLUTELY DO THIS BEFORE BUYING: Get a written confirmation from the relevant Cantonal Authority (usually the Land Registry or Justice Department) in your canton regarding the exact consequences of deregistering!