You only have three lights that worked so after that third light there is a disconnection. Because of the disconnection all readings after that in the circuit are high because there is no complete circuit. Presumably the four lights that show a low reading were the three that had been working and the fourth is the location of the disconnection.
When testing at the downlights with the switch off all readings are high but with the switch ON the readings are low, this would indicate that the fault (between whichever conductors you are testing) is before the light switch, this is slightly odd because you had some working lights which were not tripping and often the fault would be near the disconnection.
However 0.23MΩ is not enough to trip an RCD so the fault may be only partial now. I would therefore be checking the wiring in the switch to see if there is a damaged core almost touching the back box.
I would also look at light four and reterminate the connections.
Then re test IR to ensure the fault is clear and also at end of circuit do R1+R2 and compare with Rn + R2, they should be similar if the disconnection is cleared. Remember on IR to test LE, NE, LN and compare readings.
