I'd imagine chipmixer hosts their own exit node that's able to dns to their website and a lot of requests might go straight through them.
You didn't get to the point of exit node when you are visiting onion service, it happens within the network at a rendezvous point[1]. It doesn't get "exit".
[1]
https://community.torproject.org/onion-services/overview/@chipmixer it might be necessary to look at making your nodes throttle outgoing connections or disconnecting ones that look unfavourable/spammy/are big).
Umm, it wouldn't be possible due to how onion service works. If you can distinguish an user, there is no privacy/anonymity.
So what if the BTC is sitting in my session and the 7 day session expires while the site is down?
What happens with a user's deposit if the site doesn't come back online 7 days after the session expires, but he couldn't use the service or extend the session due to the downtime?
We make refund if user couldn't use the service due to the downtime. In cases of longer DDoS attacks we extended sessions time.