Both my male cats have been neutered and both have the flabby belly.  This
is pretty normal for a cat that has been neutered. As far as the marking, it
does sound territorial/behavioral.  But you may want to rule out a physical
cause, like a urinary infection (seems unlikely that both cats would have
UTIs at the same time).  Is this something that just started recently?  If
so, has there been some change to their environment, home, etc. that would
cause them to want to mark?  New person in the home, new pet, remodeling,
etc.?  You may want to try getting some Feliway diffusers and keeping them
in different areas in your home.  Also, talk with your vet for possible
suggestions.

S.

All of my normal to heavy cats do - it's less obvious on the thinner ones.
It's normal: it provides room for moving the hind legs.

Peeing everywhere can be territorial, even for neutered cats.  It can also
be punishment for the litter pans not being clean enough (some cats want a
sterile pan with new litter every time, others are happy if they can just
find a clear space).  It can also be something else entirely: Ozy normally
goes outside, but last week he went into his "I'm sick and need to go to
the vet" mode where he peed on the kitchen floor, sat in the litter pan
but didn't do anything, then peed on me.  I took him in and got treatment
for urinary infection.

Neither of my "current" cats (one male, one female) have flab, but I have
seen it in the past.  As you noted, it is not unusual -- and is not always a
sign of "fat."  In fact, it is often seen even in thin cats.  This is
sometimes known as "spay sway" or primordial pouch.  It is normal for either
male or female cats but is more common in males.  It may be a protective bit
of "flab" to protect the stomach when cats get into a fight.

MaryL