Apologies for not getting to this sooner; I’ve been busy. 
I’m pretty confident that my answer in that thread is correct:
The rationale is explained in that thread, but, essentially: the “Did you make a successful charge?” check occurs at the end of movement. While the rule could be spelled out much more laboriously to avoid all confusion, the intent – confirmed by a rules Infernal in that thread – is as above.
While the thread and original question centered around Dual Attack, the rules discussion and rationale remains the same when considering other scenarios. “The check for melee range is done at the end of movement.”
The only restrictions explained in the rules are as follows:
This is checked at the end of movement. Do you have an enemy model in your melee range? If no, you forfeit your Combat Action. If yes, you obviously must still have a Combat Action, because you’re instructed to make a melee attack.
If, when it comes time to make your Combat Action and you still have an enemy model in your melee range, then you’re required to make a melee attack.
The rules don’t provide instruction in the other case: if you had an enemy model in your melee range at the end of charge movement, but now you don’t. The model still must obviously have a Combat Action because nothing says otherwise.
At this point, the only sensible choice remaining is that the model can do whatever it wants with its Combat Action, like normal.
(Other interpretations really fall apart, by the way. The “you must make a melee attack, no exceptions” argument naturally leads to this conclusion:
- You must make a melee attack, no exceptions
- There are no enemy models in your melee range
- A friendly model is in your melee range
- You must attack a friendly model
Which is obviously nonsense. )