It would be nice if you could have a separate "space" to store the coins that we use for gambling and a "space" where you keep the coins for the "income" option. You could offer the users a drop down menu to select the "space" for the different options.
I know Freebitco.in can benefit from mistakes like this, so there is no reason why they would invest time into something like this, but it will be a nice feature for the users.
I stopped using auto-bet, because of this and I think some people might have stopped gambling all together because of previous mistakes like this. 
The other failsafe option is to have another account to keep a separate balance for the interest. As long as you don't use it to claim free rolls as well as your main account we don't mind people doing that.
As this is allowed, is it also allowed to refer the one account by the other one to get referral comissions? (Of course using Free Rolls only on one account)
No, that would be abusing the referral system. There is no rule against having 2 accounts but the referral system is meant to reward you for introducing new people to the site, referring yourself obviously doesn't fit into that. It is really just a matter of common sense as what is and isn't abuse.
The problem is, this second account might be flagged erroneously and you might end up losing your income generating account. I would not have to risk that, if I could just configure a percentage of my total balance for a specific purpose. That should not be too difficult to code?
This could be seen as a "budget" tool for beginners and you can give people the option to turn this on or off.
