hi raphael

i can answer the 2nd question ...
i do not think there is a problem at all.
the magnet just allows electricity to flow through
the spent fixer to plate the silver onto the cathode.
as long as there is enough silver in the fixer there won't be a problem
otherwise it will smoke and smell like sulphur and burn out the magnet.
when you send it to be refined, they take into account
that there is a little oxidation i would imagine that is why
they suggest the silver on the cathode isn't 100% pure silver ..

i don't really have an answer to your first question but a guess and a suggestion ..
maybe you can experiment and see if it makes a difference
if/when you use a glass container.
i save my spent fixer in black 1L jobo containers
and a 3 quart tupperware plastic container before i
run it through the magnet i have never seen the sludge you have mentioned.
maybe i don't save mine as long so it doesn't have the same time to precipitate ?

i look forward to reading what others have to say !
john