The second filter (after the electric lift pump) remains in service. Its purpose being to protect the injector pump from a potential catastrophic failure of the electric lift pump.
With the newly installed filter, it is unlikely you will ever need to replace the second filter.
/ New to me BX25, what recommended mods/improvements to do?
Yuk. Bet that was fun!
I generally leave my ZD mowers with treated fuel in them over the winter, but they usually just sit in my cargo box all winter so it doesn’t much matter. I did get stuck with some grossly contaminated fuel several years ago in one of the mowers (totally my fault). After getting that mess straightened out is when I installed the large 2 micron filter replacing the first oem filter. I never did confirm the micron rating of those plastic inline filters, but I believe they are around 25-50 microns.