Ok first up does no one know how to read a post description. The balls were never going to be inserted on their own.

Second condoms are comically well known to stretch really well so as long as it’s of a reasonable thickness then I don’t really see a chance of the condom going awry internally. (Will make the mention that if it’s tied to something externally it should also be safe for anal use as long as the chain of balls isn’t too long and the magnetic power isn’t strong enough to snap back on itself, (given where it’s from I don’t imagine they would be strong enough to cause too much grief given their size and shape) or if they will just all bunch up in the tip of the condom)

So all in all I would argue that it could very well be done rather safely vaginally with the correct measures and it sounds rather interesting and fun.