1a) This speed restriction is for departures, unless cancelled by ATC for operational reasons, theirs or yours.
1b) ATC speed overrides this. The ATC assigned speed does not automatically expire at FL100.
If you need to slow down, informing ATC of this requirement is necessary.
2) ATC speed stands. ATC might cancel the published SLP restriction e.g. "speed at your discretion" but if you have been asked to fly faster than the SLP speed it would be prudent to check with ATC if the faster speed is still needed.
3) As in point 1b.
4+5) A new clearance cancels a previous clearance. If the level restriction at or abeam WAYP1 is still required it must be repeated by ATC in the new clearance. If not, you can descend at your own rate. However, for the avoidance of doubt, it would be prudent to check this with ATC.