They can't stop you having gold or silver coins, you just have to have the requisite insurance to be compliant because they are deemed to be "collectables". There is also AFAIK no way they can legally stop a SMSF from keeping cash in a SDB. But even if you minuted it properly it would raise a red flag and you'd get the rubber glove treatment.
So far, the switch hasn't worked out very well, though only marginally. In the week since I switched, my previous fund has grown a touch, while my new fund has shrunk a little. Not enough to be a drama, but if the sharemarket takes a big tumble at some point and my new fund doesn't hold up, I'll be miffed. I wouldn't necessarily think I'd done the wrong thing, because this is all about security rather than growth, but I'll be cross if I still see falls on poor sharemarket days but don't see gains when things are going good.
It's a self managed super fund for a reason, so you get to decide how to invest the money. If your strategy is 100% cash or 100% gold then you simply document that and the reasons you think why. It could be as batshit crazy a reason as you like. You could harp on for 100 pages about the Illuminati, market manipulation, hollow earth aliens invading etc. No one gets to judge your reasons, as long as you document them then you are compliant.
I doubt it. If they use a bank to store it then the bank doesn't hold the actual cash due to fractional reserve lending. I can't imagine super funds being any different.
When you switch your super plan from higher risk to lower risk isn't there a penalty as they fund has to sell you out of the riskier investments and buy you into lower risk items?