Temp and boiler lockout aren't linked.
You said that it would take hours to reset, what was it doing when you pushed the reset. What noises was it making?
Could be anything. But nothing a decent engineer can't sort.
Loss of pressure could be expansion vessel, but should be checked and repressured with a service. Could be that the pump continues to run?
Difficult one without having eyes (and ears) on.
So control stat and high limit failed at same time? Wouldn't expect both to fail, potential wiring issue.
Hole blown in pipework? Plastic pipe within 1.8m of boiler? Failed pressure relief valve?
How did you test the pressure on the expansion vessel? If the system was still pressurised you...
I started a long winded response but deleted it. You need to back to the drawing board. Underfloor will be intermittent at best and as such is a waste of time. When its on it needs a constant feed and when its off its not really off its just set back. Depending on floor construction you could...