Are there any Winternitz alternatives out there which on the one hand make quantum computer attacks hard and on the other hand don't mess with the ability to reuse an address? Or are these properties mutual exclusive? (I can't assess this because I am not a crypto expert)
IOTA uses Winternitz one-time signatures to be more resilient to quantum computer attack vectors. The (in my opinion) very significant downside is the address reuse problem. It forces me to keep an address as a secret which does not feel right.
If for example someone knows my deposit address on Bitfinex he/she can send Iota to this address before or at the same time as me and I may loose my money because the exchange states: "After the first use of a deposit address, subsequent deposits to the same address will not be credited."
While this example may sound constructed it should illustrate the in my opinion questionable design decision of Winternitz usage in Iota.
You win on the security side by being more resilient to an attack vector which is not present as of today but you loose big times by having problems with address reuse which are very real if you think about the huge reclaiming issue.
I guess the whole address reuse issue may be one of the reasons why exchanges are hesitant to list IOTA if you think about the support tickets which arise by users reusing their deposit addresses.