It happened to me too - a day's worth of keys and no rubies. I don't really know if rubies drop rate is computed honestly for each key spent, or if there is any funny business going on. The changes in the number of rubies I get can be quite significant, so I would nto be surprised if it's not just random probability.

I avoid using all keys at once, just in case. Usually, for 100 keys spent I get 0 or 10 rubies with roughly equal chances, and rarely 20 or 30 (or even 40).

Don't know yet about the highest chair level.