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Does this mean that the operator of a solo pool has the technical possibility of manipulation, e.g. to redirect the reward to himself in case of a block find? And this although the user of his pool had entered his own Bitcoin address as user name when started the mining? I would be interested from a technical point of view whether this would be possible in general. That would be disastrous, of course, because a pool operator could be a scammer. Can you please explain this in more detail? Thanks.
Well, yes.
Your
BTC address is just being coded as reward recipient vs a pplns pool address to be split up by the pool & distributed to the users. That choice is setup by the operator.
In the case of -ck solo he coded the front end to use the address you provided to pay all but his 2% fee to you instead of running through the pool wallet first. For Kano's solo pool you setup the payment address in your solo act, the pool wallet gets the reward and transfers it (-0.5%) to your address on-file.
Either way the address you supply does *not* directly where the block reward goes. The pool operator does.