Damage is not to be discounted by tanking, but it will generally only be a make-or-break when you're looking at the bleeding edge, i.e. hard mode ICC.
What I'd expect from your build?
Before 3.3.3 the build would have lots of passive survival buffing and relatively VERY weak threat. Now? You will probably do fine, provided you use Icy Touch in high proportion.
My bigger issue with builds like this is that they are lacking in cohesive design. What do I mean?
Blood Runes = you will have a weak BS and a weak BB, neither of which will feel meaningful to hit
FU pairs = you generally won't want to use OB because it will wipe your diseases, but it will also hit relatively softly. Your DS will hit softly and heal for as little as it can. Generally, you will probably just want to use Frost runes for Icy Touch, which will give you plenty of threat, but be a bit boring. To go with that, PS will hit softly and feel like filler.
RP burning = RS is ever-present, but it will hit about as softly as you could get (without any of the passive damage buffs your threat suffers as well). You don't have FS, and your DC will not be anything special.
Overall, you have plenty of passive survival tools, but the spec will lack style and synergy that comes of actually playing to the design of a tree.
On some of your conclusions: Unholy Command has much more relevance in PvP or chain pulling instances. I liken this to glyphing taunt, if you are expecting to taunt a lot, you're trying to cover a lack of threat or poor skills at picking things up. Those are the things to fix, not just be able to taunt stuff back faster. There has been one clear place where Unholy Command has been very useful in pve: Malygos for pulling in sparks.
Mark of Blood is a great spell, but it is not an oh sh*t button. It will not save your life usually when things are bad, it is a tool worth using to ease the damage for healers in non-emergencies and is heavily worth using on CD if you have good places to use it (like Unbreakable Armor).
Glyphing Dark Command is a little silly unless you *absolutely* need for your taunts not to miss (i.e. tank swapping fights in progression for your raid team). You will normally have a fairly low miss chance if you have any hit rating, and if your first taunt misses and it absolutely can't wait for the CD, you can Death Grip. The chances of missing both are slim.
If you are looking for threat buffing glyphs, OB, FS, and DS are probably all out for the reasons listed above. I wouldn't go with Frost Fever though, despite not having a lot working in its favor, RS will probably be your strongest threat glyph. Frost Fever will only be a remotely comparable threat value when you are tanking a very large group. That said, it will support your AoE threat, since you only have DnD to really help with that (your diseases will be light, your BB will be light, and you have no HB).
Could it function? Possibly.
Will it be fun? Most likely not.
Try it out and see what you think.