This is why there's not much big tech in Germany. A single legal dispute can theoretically bankrupt any company, completely at random, at no fault of the company, but practically doesn't. It may be a low enough chance to justify investing thousands but nobody would invest a hundred million dollars in that.
That's an absurd exaggeration in regard to the issue at hand. Almost certainly far less than 1% of purchases by minors are voided, and NONE of those involve legal fees unless the seller chooses to go to court rather than refund.
In fact, I'd be willing to bet money that there are overall far less purchases refunded in Germany than in the USA.
Basically yes - the limit is generally considered to be the amount of monthly pocket money children typically get, so around 20 EUR for a 10 year old. And it would be possible for the seller to ask for a signed note of consent from the parent.
And of course the risk is limited to possibly having to revert the sale, which would be fairly rare for things that are just somewhat over that limit. Educated guess about how high the risk is for any given case are probably not hard.