What are the ways electrostatic charged objects leak charge in humid conditions?
Can airborne particles pick up charge by contact, then be repelled hence removing charge? If so would it be a significant factor?
There are two ways charge can leak in humid air. I'll give a basic description here, but I encourage you to Google around the area as you'll find lots of papers reporting measurements of conductivity of humid air and charge leakage in humid air.
The first leakage mechanism is that in the presence of water vapour virtually all materials have an adsorbed layer of water on their surface, and if this water is thick enough charge will conduct through it just as charge conducts through bulk water. See for example this paper, though note it's behind a paywall. Hygroscopic and polar materials adsorb water most easily, while low energy and non-polar surfaces like PTFE do not significantly adsorb water.
The second mechanism is that the conductivity of air increases with humidity. See for example this paper: there's no special significance to this paper other than it was the first Google found for me. The conductivity must be due to charged particles in the air, but I'm not sure it's understood what these particles are. My guess would be dust or pollen. It would be interesting to measure the conductivity of filtered air compared to unfiltered air, but I couldn't find any papers describing this. I'm also not sure why the conductivity increases with increasing humidity. I would guess that water molecules adsorbed onto the charged particles stabilise them in some way.