I think it is always acceptable for one partner to let the other know what they need in the embrace to be comfortable, period. It shouldn't matter if you're a leader or follower.
Good
However, I do not think it's okay to judge the comfort or the (restraining my own high level of irritation at this notion) the appropriateness, of the embrace from the outside. Only the people within the embrace know if they're uncomfortable. It's not my place to judge someone else's embrace if they're not the ones holding me, and they're not bashing into me with pointy elbows.
Read the OP, it's all about how it feels and whether it's comfortable or not.
Although dancing apilado there's no doubt to me that the arm should on top
of mine and reaching towards or behind the neck as far as is comfortable.
It maintains, even reinforces the chest to chest connection and positively
assists the lady to react to the movement.
/rant.
Two of you irritated now, do tango ladies have low tolerance thresholds?