Well I did a little research and to my surprise I found a whole bunch of studies that suggest Diazepam actually
increases heart rate in some cases...
So I guess I was wrong, this is really surprising to me though. I'll research a little more later because obviously benzos are widely used for stimulant overdose so I presume that, as you say, it must be that they are moreso useful for correcting stimulant-induced tachycardia rather than possessing any intrinsic ability to lower heartrate directly.
I posted a similar thread enquiring about the safety of benzos and cocaine a while back and someone quite aptly stated that benzodiazepines should still be effective prophylactics for the acute overdose symptoms that they are usually used to treat, which makes sense to me so pending a little more research I would surmise they are likely still a fairly safe combination - in reasonable doses, as ever.
As far as "your heart receiving mixed signals can cause all sorts of problems", while this is no doubt true in certain cases, for example if those mixed signals are signals to pump harder and also signals to constrict the arteries - generally the heart is not going to get "confused" by a single signal that says "pump harder!" versus one that says "pump slower!" because obviously there are no such signals, these instructions, so to speak, are instead conveyed indirectly by far more complex chemical mechanisms.
I just feel this is worth mentioning because a lot of people misunderstand the dangers of certain combinations because of vague generalisations without much explanation... such as the age old "it's bad to combine an upper and a downer", which in most cases is dangerous because of the possibility that the 2 drugs have quite different durations of action, so the combination usually only becomes dangerous when the shorter acting one starts to wear off, leaving users either dangerously overstimulated, or dangerously sedated. I would imagine that this is the same danger present with an IV speedball (plus the acute, inherent danger of IV administration of any hard stimulant - cocaine particularly).