But In short
Turn water mains off
Hose pipe on drain off
Open drain wait 2-5 mins then vent the upstairs rads and leave open
The downstairs rads will have to be individually drained either via drain offs like picture or via tubs / containers etc
System drained within reason some water...
Your original valves if the same as the first picture would of just required a nip of the glad nut quiet normal for them to weep if not used / operated for a while
As for leaking all I can suggest is to drain down to limit damage and get a plumber out as the person whose done that isn’t one