As to your second point - there could certainly be ways to address that. But the exam is a minor filter - the real selection would be via randomness, as I mentioned. If anything, the test makers would have less control over the officials than our current lobbyist system.
Step 1: Make the test extremely difficult so essentially nobody passes.
Step 2: Allow a de facto exemption for some group based on an arbitrary qualification you can control (e.g. automatic pass to those deemed to have "good community standing.")
Step 3: One of your pre-chosen candidates wins the random selection since they make up the entire candidate pool.
You’d simply design the system in a way in which the test is chosen by the population and/or by an entity separate from the representatives. At worst, you’d get the current three-part division of power system which most democratic countries are based on.
Indeed the very first objection you got was "literacy tests are racist". If you read the wiki page for it you'll see that the argument is not about whether they actually tested literacy or not (after the initial phase when "good moral character" exemptions disappeared), but rather whether it was legitimate to exclude new waves of immigrants who didn't speak good English at all (Italians, Russians ...).
Whether to even check the ID of voters in a rigorous way is opposed on similar grounds. If you can't check the ID of voters you certainly can't check their ID when they apply to become bureaucrats, and if you can't even answer "who is this applicant" then how can you answer more sophisticated questions about them?
Of course you can't. One man's merit is another man's vice. The very notion of a meritocracy is literally a sarcastic joke [1].
But the problem is even worse than that. EVEN IF if you could define a universally agreeable "meritocratic" system, you would still have the problem that people with power would abuse the system to lock out others.
Suppose 100% "meritocratic" people were selected at random. They then immediately change the rules. Now 99.99% of people disagree with the legal definition of "meritocratic". But tough shit, cause this ain't a democracy, and the other 00.01% of people have all the power.
I don’t see why this is controversial. It’s no different than “fair” or “justified” or innumerable other values that a society has to work out a definition for.
I think the point of the comment you're responding to is that it's not defined by the society implementing it, but by those in power. So whether the idea of 'merit' changes across time and culture might be true, but is beside the point.
Things like this happen sometimes: