Just for the sake of argument, even if we did choose to interpret ἀμφίβληστρον as meaning 'getting tangled', or something along those lines, it would hardly prove that Jesus chose them for that reason.
Nevertheless, it's true that the disciples show an obvious stupidity whenever they open their mouths; and their feeble-mindedness is by no means limited to the Gospel of Mark. The only disciples spared, I suppose, are those who never utter a word.
I think the most straightforward explanation for this was to make Jesus appear wise and intelligent by comparison. His counterparts' loutishness takes its fullest effect, IMO, in these passages:
Matthew 16: 5-12 When they went across the lake, the disciples forgot to take bread.
“Be careful,” Jesus said to them. “Be on your guard against the yeast of the Pharisees and Sadducees.”
They discussed this among themselves and said, “It is because we didn’t bring any bread.”
Aware of their discussion, Jesus asked, “You of little faith, why are you talking among yourselves about having no bread? Do you still not understand? Don’t you remember the five loaves for the five thousand, and how many basketfuls you gathered? Or the seven loaves for the four thousand, and how many basketfuls you gathered? How is it you don’t understand that I was not talking to you about bread? But be on your guard against the yeast of the Pharisees and Sadducees.”
Then they understood that he was not telling them to guard against the yeast used in bread, but against the teaching of the Pharisees and Sadducees.
Luke 9: 52-54 And he [Jesus] sent messengers on ahead, who went into a Samaritan village to get things ready for him; but the people there did not welcome him, because he was heading for Jerusalem.
When the disciples James and John saw this, they asked, “Lord, do you want us to call fire down from heaven to destroy them?”
But Jesus turned and rebuked them.
John 11: 11-14 After he had said this, he went on to tell them, “Our friend Lazarus has fallen asleep; but I am going there to wake him up.”
His disciples replied, “Lord, if he sleeps, he will get better.”
Jesus had been speaking of his death, but his disciples thought he meant natural sleep.
So then he told them plainly, “Lazarus is dead."
Mark directly informs us that the disciples have difficulty understanding his parables.
Mark 4: 10,13 When he was alone, the Twelve and the others around him asked him about the parable. Then Jesus said to them, “Do you not understand this parable? How then will you understand any parable?"
Mark 7: 17-19 After he had left the crowd and entered the house, his disciples asked him about this parable. “Are you so dull?” he asked. “Don’t you see that nothing that enters a person from the outside can defile them? For it doesn’t go into their heart but into their stomach, and then out of the body.”
Note that in each of these examples, the disciples' remarks are met by Jesus either rebuking them or pointing out their stupidity. This a simple method of making him appear wiser, albeit less kind. Evidently the authors thought it more important to emphasize his intellect over his kindness.