Lets not forget that underclocking your cpu creates the same instability
that overclocking does.
Generally, no, it will typically INCREASE the stability
margin, make it more stable than at "normal" speed.
"IF" there are other issues, they could effect stability
underclocking. Synchronized PCI bus speeds, lower/faster
memory timings, or manually lowered voltage could introduce
instability but these are special cases not implied by
underclocking itself.
Mind you, you can probably do so safely up to 10% as
you can with overclocking
Completely invalid statement.
It's quite easy for anyone to underclock their system less.
Practically any system with the jumpers or bios settings to
allow FSB decrease can be underclocked stabily by more than
10%.
Your random guesses are out of place in a technical group.