Now I get your Username, heehee
I agree on the fine METAL canister filter being on the pressure side.A clean new sock and a coarse prefilter have kept my carb clean for over 15 years.The sock is huge, comparatively speaking.
And Varnish is about the only thing that clogs it. So long as you do not allow the fuel to evaporate from the tank,completely,chances are very good that it will last for decades.But if you put a strong solvent in there to expel the varnish through the system, and to be consumed by the engine;then you run the risk of the varnish collapsing into the bottom of the tank in sheets, and it's game-over.
Going back to the open vent line at the canister; atmospheric air gets pumped into that tiny line whenever there is a temperature change around the line. That means day to night and season to season.Of course as the liquid level in the tank falls, this is accelerated. Atmospheric air has moisture in it. Moisture condenses to water, which when in contact with steel and in the presence of oxygen, reacts to form rust.Rust doesn't care about up,down, or sideways,so in that confined space it quickly grows to fill the entire space; usually in the lowest point in the system.