No choice for me, I'd do all 4 - a great diving itinerary.

If you're looking to do only 2 to cut costs, you could do shore dives at
Roatan and Cayman (Cayman - turn right after walking down dock - see
Eden Rock in less than 5 minute walk / Do a boat dive scheduled thru the
ship at Belize (only way the ship will wait if you are late returning
and most of the Belize excursions have that possibility), and in Cozumel
book your own boat dives with one of the many ops.

If you're limiting your diving because you have a SO who doesn't dive,
they will likely be happiest left alone with your credit card for a few
hours in Cozumel and Grand Cayman.

How they typically work, YMMV - sign up on-line before you go or as soon
as you get on the ship - cabin staff will leave your excursion vouchers
in your room, it will have meeting details on it - meet at appointed
time in appointed place on the ship - show C card to dive staff - wait a
half hour - go to dock/tender as a group - wait 20 minutes - get in van
or boat to dive shop - fill out forms and sign waivers - wait 30 minutes
for everyone to get equipment - get on boat with fellow divers (minimum
requirements are one dive in the past 3 years in some cases) - dive
30-45 minutes - SI 30 minutes - dive 40-45 minutes - return to shop -
wait 30 minutes for everyone to rinse gear, return equipment rentals and
buy t-shirts - get in boat/van for return to dock/ship.

Best bet if you are an experienced diver is to book your own dives
(saves a few bucks as well) or buddy up for some shore dives.
Just remember, the ship can be late arriving or change itineraries at
the last moment, and it won't wait for you if you return late.

If you have time constraints, or have limited dive experience go with
the ship offered tours - the ops the ships use are generally safe and
attentive to new divers since they see quite a few.

Whatever you do, make sure to take some sort of tour itinerary for Belize.
I don't believe you will particulary enjoy hanging around Belize City.

As for diving Belize, do you know which site they will take you to?  If it
is Glover's, then I think you should go for it.

Roatan is a lovely island, though very third world for the most part.  I
enjoy the diving, mostly because of the undersea topography.

Coz is great diving.  I haven't been to Cayman.

The Valor is a great sampler cruise.  You get four of the major stops
in the Carib.

For your first cruise, and since you may not know the islands (if you
do, ignore this), I'd recommend you book through the line.  You don't
say now experienced you are, but that may be a major factor in your
choices.

You arrive Belize 9 am, leave 6 pm.  In theory, plenty of time.  But
it depends on when the dive operator leaves the dock.  This is a
tender port so timing may vary.

Roatan arrives 7 am, leaves 3:30 pm.  Might be kind of iffy with an
8-8:30 dive boat departure.  It depends on where they go.

Grand Cayman arrives 9 am, departs 5:30 pm.  This another tender port.
You may not reach the dock until 10, unless you have a ships excursion
(same applies to Belize).  This is probably the least iff of these
three.

Cozumel arrives 11 am, leaves 11 pm.  Plenty of time, if you can find
somebody who does an afternoon dive.  Plenty around, others can
probably suggest one.  And you can still get trashed at Senor Frogs
and crawl back to the ship.

I also recommend flying in the day before, especially if you are
flying from anywhere in the north.  If your flight is delayed, you may
find yourself waving at the ship.