やら is definitely very similar in meaning to か, but, as you might expect from a particle that's not etymologically related and thus developed separately, there are distinctions in nuance and usage that make it hard to call them perfectly equivalent.
The main semantic difference between the two is that やら provides a certain deliberate emphasis on the speaker's uncertainty about whatever it's attached to, whereas か is neutral and objective. You can think of it as something like an implied 分からない included in the particle. The most obvious implication of this is that やら can never be used to state an actual question in the way that か can. 何をしているのか can be a direct question "what are you doing?", but 何をしているのやら can only be a rhetorical one - you can almost hear an unstated さっぱり分からないね following it.
In situations where か would not form a direct question, the implications of this emphasis on uncertainty are more subtle and can vary somewhat. In many cases the choice of やら implies dismissiveness, ie. that you're leaving a question open because the answer is unimportant. For example, while 彼は何か(を)書いている is a very neutral statement that "he is writing something", 彼は何やら書いている sounds more like "he's writing something-or-other" - there's a distinct implication that the speaker doesn't care what he's actually writing, or can't be bothered to find out.
In other cases, the implication can be more straightforward unfamiliarity. For instance, in fairly common expressions along the lines of 何やら面白いことになっている, the choice of やら certainly isn't dismissive per se; if anything it indicates curiosity, the idea that the speaker can't imagine what is going on but it certainly seems interesting. If it were 何か面白いことになっている the sense of uncertainty would be weaker and less personal, so this form doesn't see as much usage.
Outside of this added subjective nuance, though, I would say that やら and か are very close in meaning. Even the "other forms" you listed could be reasonably substituted with か in one way or another. Something like 田中とやら "Tanaka or whatever his name is" is roughly the same in meaning as 田中とかいうヤツ - the latter can similarly be abbreviated to 田中とか, but the dismissive nuance of the phrase is better suited to やら, so it's that form that has fossilised more recognisably as a set expression. ～やら～やら as a way of listing things is essentially equivalent to ～とか～とか, with the choice of やら providing only an added sense of not caring much about the particulars. And ～やら "I wonder" is, as I touched on above, simply the result of substituting やら for か in an actual question, thus making it strictly rhetorical.