I don't think anyone was recommending Kings Cross to Watford (Met). Rather it would be either walk or tube to Euston then LM to Watford Junction.Try XC Doncaster to Birmingham and LM to Watford from there (out of curiousity) - KX to Watford (Met) by tube is not reccomended on time grounds (45+Mins)
I'm not sure where Watford (Met) has come from! The OP did say "Watford" in the title - I will now correct this to Watford Jn to avoid confusion, however the detail did make it clear that the journey is to Watford Jn and the advise has been given accordingly.Try XC Doncaster to Birmingham and LM to Watford from there (out of curiousity) - KX to Watford (Met) by tube is not reccomended on time grounds (45+Mins)
Ah, the LM anomaly (AIUI). Note that it only applies from Euston NR. Both peaks apply from Euston LU (not that you'd easily make the journey from Euston LU).The Oyster PAYG fares on this route are a bit odd, as customers are penalised for some journeys if going from King's Cross, rather than walking to Euston, but not always. If walking to Euston, there is no morning peak from Euston to Watford High St/Jn.
The validators ARE continuation exit at Harrow & Wealdstone. To split a journey in two you would need to go out of either gateline and re-enter.(It can be cheaper to "split" at [stn]HRW[/stn], however I am unsure how this could actually be achieved, I know there are readers on the footbridge but if they are continuation exit, then I am unsure how you'd split it into 2 journeys)