You keep interchanging 10% crit with 10% damage. I'm not sure how that really calculates out - doesn't make much sense to me.
The math is simple, as is understanding the value of crit for increasing damage. With 200% crit damage (everything but HS and DS if you have MoM), 1% crit = 1% increase in average damage from that strike. How much crit you have before or after is inconsequential in that regard, so saying "10% crit on RS when you're a tank and have low crit chance," is a faulty statement. However, you are right that RS is a move worth buffing. RS gets a 150% threat modifier that none of the other strikes have, so for all intents and purposes you can say a 10% increase in damage is a 15% increase in threat on RS (note: a 10% increase in damage is a 10% increase in threat otherwise, as the modifier from Frost Pres applies the same to every other move). So, the question for me, as I am quite fond of Disease, is do I want DS or RS glyphed. In order to compare, you only have to look at the actual applied buff. DS gives +25% damage = +25% threat to DS, and RS gives +10% crit = +15% threat to RS. In other words, in order for glyph of RS to be better than DS for me, if my DS threat is ~18% of my total damage threat, RS would have to be 30% of my total threat in order for them to break even. Usually it's closer to 25-27%, so DS wins out. I've done tests to support my math, and the math holds true. There are many factors as to why this is, and I make a point of saying, this may not be true for everyone. It is true for me (to my surprise despite the 2 pc bonus benefiting RS) because I DS a lot, not everyone will DS is much as I do.
Perhaps you need to brush up on your parry-haste mechanics? Every time YOU parry, the speed of your current attack is increased, meaning the more you parry, the more often you will strike. Meaning more than 20 swings per minute. I was not a DRM user before (because I was using WotN) so without that mechanic as well, you'd definitely be looking at fewer HS than RS with my previous spec, especially on a faster hitting encounter, or an encounter where you are tanking multiple enemies. With every single skill having Rune Strike macro'd into it, I have always put out more Rune Strikes in the long run. That might change now that I have DRM, I'll keep an eye on it.
Here's where you have gotten carried away with yourself. You buy into your own assumptions. Again, if you read above, RS is my 4th favorite glyph, and I made tough decisions leaving it off. I don't regret it. You value RS but fail to see the value of Subversion. Subversion increases the crit chance of HS and that's all it needs to do. Remember how I said 1% crit is 1% average damage/threat with 200% crit strike? Well with MoM HS gets a 245% crit strike. That means that each point or 3% to crit, is a 4.35% increase in HS damage/threat. Here's where results may vary. For me, HS does slightly more threat than RS, and so, increasing that by 4.35% (per pt!!) is a no brainer the way you consider RS glyphing to be a no-brainer. Also, as a side note, check your combat log parses, if RS is firing more often than HS, you're doing something weird. Just off the top of my head, with a typical 2h speed, after raid buffs, you'll get about 20 swings per minute. If you turned 3/4's of them into RS, you'd get 15 RS per minute. In a minute you will go through 6 rune sets. I average about 3.5 HS's per rune set, which is 21 HS's. But who knows, maybe you have a different combination of things that let's you fire it off faster?