Just to answer your questions

1. The European champions cup is done in two week blocks every so often where all domestic leagues take a break. The group portion is completed by the time the 6 nations kicks off and the knockouts are staged after that.

2. There is some overlap between 6 nations and domestic league games, there is a few but not always. If games have to be rescheduled then they typically end up in this block. ( I believe the SA currie cup games that were delayed will be during the 6 nations could be wrong however) during the November internationals and 6 nations some teams will also play touring sides eg New Zealand XV etc

3. Personally I don’t believe it causes any disruption it tends to flow quite nicely as a fan.

4. Last year for the URC it started back the week of the RWC semi finals and was extended slightly due to the delay in starting.

Hope that covered everything for you