Below 700' on an ILS the system memorises the VAPP target (incase of a dual FM failure with managed speed) and to stabilise speed for the approach. I assume that, as the surface wind entered into the PERF ARR page vs. Current wind determines the GS MINI variations in managed speed and therefore has an affect on VAPP, The PERF ARR page is locked as a result.
LAND mode indicates that LOC and GS are locked and no actions on the FCU can alter that. The only way out of LAND mode is by go around, both autopilot/FD off or pressing the APPR PB after the aircraft has been on the ground longer than 10 secs. This is to stop inadvertent detuning of the ILS at low levels. Basically, if you have the LAND FMA and you don't get the red Autoland light, you will Autoland safely