I live in tropical north Australia with humidity levels of 90% for a couple of months at a time and during this time it is non riding time so I have several leather saddles and bridles stored inside and inevitably they get some mould on them. Plus of course my shoes and handbags!!!
Keeping them really clean helps, plus the silica sachets in and nearby.
I was doing some today, a chamois cloth dipped in a half bucket of water, with a splash of Dettol, wring the cloth out till it is just damp and wipe really well, rinsing the cloth in between if needed. Then let air dry, maybe under a fan.
When the leather is super dry I slather leather balsam on, BUT I am sure that is not for your fabulous handbags.
So you would then replenish the leather with whatever your manufacturer recommended.
The Dettol or Oil of Cloves kills the mould spores, like the sun does, but the sun is very drying to leather.
If you're unsure of this method, just try a little on a place you can't see, especially on highly coloured leathers.
I have never done it on super expensive handbags, I don't have any like that unfortunately, so I can't guarantee you anything as far as they go, but I do my normal bags and shoes.
Just make sure you use tiny bits of Dettol etc and use a chamois cloth and wring out well, you are hardly wetting the leather at all, basically just picking up all the mould spores and killing them, unlike brushing which just relocates them