I think they have a lot of levers. There are many default strategies with a split between stocks and non-stocks. They could easily throw these MBS into the standard pre-defined strategies.
For 3a cash, they could do what a bank does, take your deposit money, give you tiny interest, invest it in the MBS and earn the difference.
Are you sure? If I deposit CHF 100,000 into a normal bank account at (for example) Raiffeisen, Raiffeisen uses the money for banking business, i.e. investments, mortage, etc. no?
What you write is exactly what I worry about. Fom a user’s point of view, if you don’t need a mortage yourself, both sound like a conflict of interest. This creates the same problem for Finpension that every traditional bank has (i mean, they want to become a bank, so that’s just the way it is): suddenly, you have other products that you need to sell. And the easiest way to do this is to sell products, that may eventually turn out to be bad (we don’t know), to existing customers.
And if Finpension can use (I don’t know if they can do that, hence my question) existing 3a account money lying around for this purpose, then this happens automatically.
Don’t get me wrong: I have confidence in Finpension. But you still have to be careful. It’s still something like a financial start-up, which of course can fail, like every company/bank.
Every institution has gone along this route. Even supermarkets in the UK: they famously have negative working capital requirements because they collect money at retail but pay suppliers late. Many of them ended up offering financial products such as mortgages and credit cards to soak up this cash asset and profit from it.
I strongly hope that supermarkets in the UK are not comparable to Swiss banking regulations 
True. Swiss banks are probably more often in trouble. See UBS and CS 
Yep they cannot touch your money. They create money out of thin air to make profits and have positive balance sheets. The payment system is separate to banking and is kind of a public service that banks provide.
However there is a 100k deposit guarantee scheme in case a bank goes bust, this is because banking and the payment system are intertwined.