The intent of the scene is clearly that they are stopped just out of bow and Scorpion range, and only Tyrion approaches close enough to be in danger. It just utterly fails at conveying this visually, possibly because they also need to be close enough to see Missandei get killed so the range of the Scorpions magically shrinks.

The obvious answer is: Jon Snow. She knows he's in league with her, and he'd simply show up and continue the war without her anyway. And if she's heard anything from the battle in the North, she'll know that Jon's ridden a dragon before, so Drogon would still be a threat (unless he got emotional and exposed himself).

Secondly, killing the enemy commander during a parlay is a good way of making a martyr out of your enemy. That makes it a lot easier for the enemy forces to double down on their resolve to take you out. And it makes it harder to cast yourself as the benevolent protector she wants the people to think of her as. Cercei's going out of her way here to make Dany look like the crazed, murderous aggressor, so that even if Dany fez win it'd be a poisoned well: the people would hate and fear her.

Thirdly, Dany's no good as a military commander, as should be quite obvious by now, so you wouldn't make much of a dent in their strategic capabilities, and may make them melhor by getting rid of her tactical dead weight.

That's not how politics work.

Remember the previous season, Cersei meets with Jon & co. and they show her the zombie they stole from NK. At that meeting Daenerys had both her dragons and there were no scorpions. She could easily burn Cerse, Mountain , Jaime...

Mas

If she did that , she would be an unfaithful ruler and everybody would be afraid to meet her in person from then on.

Same is true with Cersei

If she basically kills dragon or the others, she would be a queen who does not respect agreements. Everybody, friend or foe, would be afraid to go to King's Landing and her power would diminish, because people would lose their trust to her hospitality.

Real Life Politics

This is also true for real life politics. Country leaders visit other countries all the time and they do not get killed. Remember , They don't always visit countries they are allies with. (e.g. USA and North Korea , any UN meeting...)

Durante o grande tiro do topo das muralhas, você pode ver que o dragão está muito mais longe do que o resto da guarda de Dani.

Presumivelmente, está fora do alcance da escorpiões. The show doesn't make it clear how intelligent dragons are. When they are allowed to roam, they seem to behave like cats (they come and go as they please). It's not clear if they are less intelligent, more intelligent, or as intelligent as humans. Certainly, Cersei would not know it.

For all she knew, if she killed Dani, the dragon would wait for his chance.. maybe for weeks or months. But then it would attack at night and burn down the city.

This may answer the question why Cersei had to kill Missandei. Just like in the Battle of the Bastards (when Ramsey killed Rickon), she needed to kill someone Dani cared about. This was done to draw the strongest forces into the fight.

Cersei can take on a dragon when she is prepared for it and has all the strongest defenses with her. She is not likely to fare so well if she is unprepared and the attack can come anywhere, at any moment, from a being which travels much faster than any of Cersei's own forces.

The look on Cersei's face after Missandei says "dracarys" is that of a self-satisfied smile. Her plan is working. She would much rather fight the fight where she is the strongest than have to fight against a guerrilla force with a dragon.

A parlay is risky for both sides. She Cersei had attacked she would be risking retaliation. She was on top of the walls in a vulnerable position. Maybe her forces could have taken out Drogon in time, maybe she would have been incinerated.

It's also possible that she simply intended to anger Dany, in the hopes of her making further tactical errors. Dany has not shown herself to be a good tactician and many of the set-backs she has experienced have been due to her own inexperience.