There is no requirement to install 8.5 before X1. Read the update notes for X1, there is no need to install all the updates, some of the updates are cumulative. The only time Cakewalk required a previous version of SONAR installed was the download upgrade from 8.0 to 8.5. All other upgrades have been full retail versions of SONAR.
Cakewalk has not said either way. I do not believe there is a rush write new installers for legacy products. Cakewalk has recently updated installers for DPro and Rapture expansion packs. That may be due to the release of RPro.
I looked at my legacy products page and most everything has a serial number and registration code. All the credentials needed to install should be listed with each items. There are some items that only require a serial number.