He wanted a piece of gum.
Also illustrates his sleazy side quite well, just gives a bit more to dislike about him.

Well time would have been of the utmost importance to get somewhere out of sight, such as Kitty's dark apartment.
They picked the lock in a lighted hallway.
Once inside, they sat quietly in the dark so that she did not suspect they would be in there. They would not have waited long, the night had been filled with much action as it was, so it was getting late. Kitty is a lowly reporter, not as if she'd need to be working very late in the night.
Have you ever picked a lock? Not easy in the dark and awkward when you are attached to another person. Much easier to wait until the lights are on.

We cannot say for certain how much time there was. They had to get there first, pick the lock and get inside. Now they have just escaped custody, ran around a few streets etc and ran in front of a bus. Kitty is a reporter with a more than passing interest in what Sherlock is doing, wouldn't you agree?  I am sure that had she known Sherlock was arrested and then went 'on the run', she wouldn't be toddling off home for the evening; she would have been out  and about trying to get the dirt on this latest development.
But she wasn't. She too has a mobile and would have been alerted asap. But the story hasn't got to the press yet, so not very much time at all had passed between their escape and her arrival at her flat. So I would be quite happy in saying, they hadn't had time to pick the lock on the handcuffs yet.
Using a phone would still be a light source that could have been seen from outside. It is not as if they knew the layout of the apartment, so hard to tell what can be seen from outside.

No.
The other victims were 'lost' or 'late' or in some way willing to put their fate in the hands of a cabbie during a few moments when they needed their help.
The tourist was simply a straightforward fare.
Businessman was running late, the young guys was stuck in the rain, the lady in pink was wet & bedraggled and wanting to get to a hotel fast.
'Jeff' preyed on those who reached out to a stranger for help in some way.

Sherlock Holmes wrote:

What about the woman at the party. Why did she get a cab?

I knew there was another one. Because her 'minders' took her keys from her so she couldn't drive.

I think so. That was his reasoning in the unaired pilot and I think it carries on in this version also.
To him, they are 'stupid/weak' so the perfect targets and expendable.
Someone in a strong point in their life would not be as vulnerable.

That was so corny; but funny.

It made me think, I've never laid eyes on a real gun, as old as I am! I have no interest in ever doing so. So I doubt I would know a real one from a fake one anyway!

A tourist on their own would be quite vunerable though. They wouldn't know the city, hence the cabbie could conveniently wait for a while at the address Sherlock, via John, had texted him All the people were on their own. The business man, the teenager, the politician, the lady in pink.

That's true. Just testing an hypothesis to destruction, me. Although the tourist & cab did go straight to a uniformed police officer after this incident so it would be unlikely that the cabbie, if he had an intent to play the game with the tourist would have cancelled his plans after this, especially as he recognised Sherlock.