The risk thing: Thinking again about it, I bet yes, it would be playable. Linking the "contract for the job" thing with the quests would be cool, but hardly ever I do a quest without interruption, so I think that would make a way for players to contract mercs forever paying a fix price. Like, I could hire a merc with Deadly Risk involved (so he wouldn't run away unless he's not honorable) and don't do the quest, he'd keep following me until the quest was done and I already payed him. Programming them to think "oh, it's been a long time, I should be home already, this guy is making a fool outta me, I'll go now" would be difficult I think, and unnecessary since the "contract for the job" linked with "X days" could work just as great and without much trouble for the modder.
Mercenaries could also be a faction, within which you would have a reputation based on how responsible you are with paying them, and/or how many of your mercs come back alive. (How well you treat them.) Instead of being unlimited, I think there should at least be a limited source, replenished over time in regions. There could also be a 'put it on my tab' feature for the highest reputation possible, where you could pay off some of the bill over time. Lowest rep gets you increased prices all up front. Mid range would be half now half afterwards sort of thing, with the worst possible reputation causing the whole faction to be hostile towards you, even seeking you out at certain times.
I gave an idea close to that on Steam Workshop. To make a Mercenaries Guild, placed all around Skyrim. With specifics skills for each Merc type (according to their location). A Mage Merc office in Winterhold, for example. An Argonian Merc officer, with well armored argonian dudes (since they're known for their accounting, they would have the coin to purchase nice armor, and they would be much more expansive). Making it like they would chase you depending on your rep with them is nice too.
The timing aspects could become complex as well. In some cases it can take a day just to travel to a location, so I'd think there would need to be a way to pre-pay for more than a day in advance. The other aspect is, these people don't have watches, so while the game can tell exact time, I'm not sure they should just demand more money the instant 24 hours is reached. Especially if you're exploring underground, how would the merc know what time it is? So maybe they'd stick it out with you, until you reach an exterior cell. Or maybe, when you get close to the time they're due to quit, you generate a random number, and have them ask for payment after that many number of extra minutes...maybe a little before the exact time, and maybe a little after, to add more of that element of unpredictability.
I liked the timing idea on their demanding. But let's be more clear here: I was thinking about two kinds of contracts.
1. Contract for the Job - you get to choose X days and the risk of the job (here you could do the "hey guy, stay around X more days" with a persuade dialogue when their time to quit is close, like said in the quote), the price would be made from that, and would not have a daily cost, since it's already arranged to be X days.
2. Normal contract - pretty much like it is now. you pay the price, but would add the daily cost.