As PP says, iron in extremely low concentrations will inhibit the performance of zinc anodes according to research carried out by USA military. It is safer not to use steel tools but by all means use other metals, e.g. a brass wire brush.
Yes indeed it was. Painting a propeller reduces the area of the cathode, prolonging the anode life. Since painting my Bruntons the prop anode life has doubled, 1 - 2 years and the shaft anode probably more, 2 years to more than 4.
Surely if they're only a little worn then you wouldn't be expecting them to massively decay over the next season? Like, if 25% had gone in year one it'd be really weird for the next 50% to go in year two and for a few extra weeks to make a difference?
Unfortunately "wear" is not necessarily linear, particularly if the boat moves around and is moored in different locations. similarly, if the galvanic action is reduced by painting the cathodic item and the paint breaks down, action may increase. Given the relatively low cost of anodes compared with both other maintenance costs and haulout charges it makes sense to err on the conservative side and replace the anode at scheduled haulouts.
If an anode is 25% gone....and you do annual haul outs...I would definitely replace it. 50% is generally considered finished....so it doesn’t leave you with much leeway.....anything could happen, for example a stray current in the water or a steel boat ties up next to you.