Firstly I understand and agree with you about it being an issue if it had 100% uptime as you have pointed out. However, there is a very easy solution to resolve that concern and that would be that you either ensure it won’t have 100% by giving it an internal cooldown which prevents it from triggering for a period of time. Alternatively when triggered it puts a durational debuff on the DK which prevents the ability from being triggered while that debuff lasts. Either of those options would resolve your concerns very simply without much effort.
Honestly you can’t make that assumption as right now we do not have total clarification on how this all works and this means that this statement likely isn’t totally correct.
Sure the base ability has a chance to call one of the horseman to aid you for 10 seconds every time you spend Runes. However, right now we don’t fully understand how it works or even how it chooses which horseman is called. While it could be random it might also be sequential or something else that we actually don’t know right now. This means you honestly can’t make any assumptions if each horseman has an internal cooldown for example or if it requires a condition like the others have to be spawned before it resets and can spawn again. For all we know it could actually allow them to be summoned whenever they aren’t active and if they are active they are excluded from the trigger event decision at the time. This potentially changes your assumptions about how often it might occur to be completely different. However, you might be somewhat correct as well the whole point is we don’t fully understand the conditions around this ability at this time.
In addition you have also clearly overlooked the fact that there is a talent which actually increases the duration that the horseman are active. Fury of the Horseman - right now increases the duration of each horseman by 1 second for every 50 Runic Power spent stacking up to a maximum of 5 seconds so now we have a situation where it could be more than 10 seconds and maybe even up to 15 seconds potentially.
If you also look at some of the other talents it does appear like there is a somewhat reasonable chance that the DK will potentially easily have 2 or more active fairly often as the talent Feast of Souls only works if the DK has 2 or more active.
Finally there is based on the talents options once again that a Death Knight will have all four of them active for a duration of 20 seconds as the talent Apocalypse Now causes Army of the Dead and Frostwyrm’s Fury to always summon all four when they are used. The question however is this duration also potentially affected by Fury of the Horseman and if they are then that could be up to 25 seconds.
Just something to consider