Took me ~2 hours, but I worked it out on my own. For anyone else wondering.
Firstly, check out this post on the Bioware forums: http://social.bioware.com/forum/1/topic/105/index/2933016
All of the information used in this post is drawn from there.
Minimum of 8 Squadmates to unlock Omega-4 mission.
Squad includes loyal Kasumi.
Squad includes Zaeed.
Don't let anyone die.
From my calculations, it is possible to assure the survival of all brought squadmates, as well as the Normandy crew, with just over half the squad loyal to you (8 squadmates, 5 are loyal):
The following 6 characters are required for no deaths. Period.
Mordin -- unlocks science lab and ship upgrades
Tali -- unlocks ship shield upgrade
Garrus -- unlocks ship guns upgrade
Samara/Morinth/Jack -- biotic for "The Long Walk" in the collector base
Miranda -- with you by default
Jacob* -- with you by default
*(If you recruit Samara/Morinth, and you will want to if you're striving for the minimum, you don't actually need to research the armor plating for the Normandy. Jack is not part of your crew, and so she cannot die when the hull is breached.)
It works like this -- stick loyal Tech specialist (Tali/Legion/Kasumi) in the vents and a loyal fire squad leader as the distraction team (Miranda/Jacob/Garrus). Then pick a loyal biotic (Jack/Samara/Morinth) and another fire squad leader (same as before). Lastly, pick a loyal (any) squaddie to escort the crew back. If you made these "ideal" choices (remember, they have to be loyal!) no one will die.
The last two "death checks" are "holding the line" and the escape from the final boss. The latter is simple -- if you take loyal squadmates to fight the Human-Reaper, they will survive. If they aren't, they will die. It is the "Holding of the Line" that is the most complex, and which necessitated both math calculations and prompted this question itself.
Every squadmate in the game has a "hold the line" score, either a 3, a 1, or a 0. Being loyal increases their score by one. If the average "hold the line" score is > 2.0, no one will die. Anything less will result in one OR MORE deaths.
In your case, the situation is as follows:
Tech Expert: Kasumi
Squad Leader: Jacob or Miranda (I picked Jacob, it's preference)
Crew Guide: Mordin
Human Reaper Fight Squad: Kasumi and Jacob
This leaves the following five squadmates to "Hold the Line":
Tali - 0
Garrus - 3
Zaeed(Loyal) - 4
Miranda - 1
Samara(Loyal) - 2
2+1+3+4+0 = 10 / 5 = 2.0 = no one dies.
Without your restrictions on Kasumi (If you didn't want the Locust, or wanted to do Tali's loyalty mission), it is possible to reach the absolute minimum: 8 squadmates, only 3 loyal.
Tali is both your techie AND escorts the crew back. Take Jacob/Miranda and your biotic specialist into the last fight leaving the following 5 to hold the line:
Mordin - 0
Garrus - 3
Miranda/Jacob - 1
Zaeed - 3
Grunt (instead of Kasumi. Don't recruit Kasumi) - 3
3+3+3+1+0=10 / 5 = 2.0 = everyone survives.