I read the conditions imposed by Switzerland to see how to sneak through... and there is no way without a risk.
A test as Laurens suggests is not admitted as a proof you’re Ok. They will put you in 10 days isolation anyway. It’s written in the “Swiss rules” for Covid19 people entering the country.
They say that if you’ve being in Spain in the past 14 days, you’ll be isolated for 10 days. But we’re supposed to come to Switzerland driving from France.
I have a Swiss passport and Ana María has her Guatemalan one. So, we could get in has Swiss and Guatemalan, but the 3-Wheeler still has obvious Spanish plates that may alert the Swiss neurotic policemen and citizens.
If we get in with the 3-Wheeler from France, different scenarios are possible.
Scenario 1: I guess we won’t have problems crossing the border as they usually don’t do any controls. But if they do because the situation with the Covid19 is what it is, and they see our Spanish plate in a striking 3-Wheeler, we’ll have 99% chances to be stopped. And probably our entrance rejected.
Scenario 2: we make it into Switzerland without being stopped at the border. Arrive to Grindelwald. We park our 3-Wheeler and register at the hotel with our Swiss and Guatemalan passports. Guatemala is also banned, so this might pull the trigger for the person at the front desk. If it doesn’t pull the trigger, maybe later a local bored and enthusiastic Swiss policeman sniffing around the 3-Wheelers spots ours with its Spanish plates. And starts investigating and asks questions and… OK just imagine. It’s Switzerland and I know the guys (remember I’m Swiss too, no offense). Some can get paranoid and pursue us, and we can get in big trouble then, as they can penalize us with 10.000 CHF each! Really expensive holydays then!
The problem is how would the Swiss authorities know if we’ve being out of Spain for at least 14 days before entering Switzerland? We won’t be able to prove that. They can’t prove the opposite too, but we are not the authorities, they are…
Scenario 3: we’re in. Nothing happens. We’re happy and enjoy the Junfrau-Treffen with all the gang.
To be honest, unless the Swiss change their mind about Spanish travelers, we won’t take that risk. Sad, but this is our reality today. I hope that before the end of the month the situation changes for good.
but if there is a chance we join their gang, we'll try to make it happen this year. 
