Sacred Haven of Debauchery

Of course.

So, you can't just leave "whenever you want" – because you need to maintain a blockade of the planet to preserve your military advantage. You won't withdraw your fleet "whenever you want," but when you are forced to do so due to military necessity. And if control over the planet is established, then yes, you can leave whenever you want. Thus, a planetary blockade is a very fragile and temporary achievement.

No. The only question is the presence of anti-aircraft guns. In Stellaris, they decided that this is not possible against orbital attackers. How realistic this is is another question.

The question is the presence of organized resistance. An administration that has full and actual control over the planet (in the case of the Hive, it is nominal due to various clans). A chain of military command that forces ground forces to act in an organized and coordinated manner (and not as a collection of field commanders). Finally, the presence of various bunkers for the civilian population, etc.

Well, one raid – yes.

No. The counter-terrorist operation in Chechnya lasted almost ten years.

I would say that, no, it is of little use. And I don't think ground forces feel it, probably not. There's just a lot of politics involved, and they can't figure out who the enemy is and who the friend is. Anyway, with a clear goal and motivation, World War II showed the effectiveness of bombing quite well. A decisive factor? Well, not necessarily. But it is one of the important ones.

Damn, just say it already – are ground troops in the space age still cavalry with sabers, or are they a necessary element, where air superiority acts solely as support for ground forces? Because cavalry with sabers is not.

A strange statement, considering that developed countries usually fight each other in such cases. And when they don't fight each other, they either quickly achieve their goals (Iraq, Libya – the governments were changed to those that the United States needed. More precisely, to those that no longer show direct hostility), or they simply lose interest because there is nothing more to do there (well, they can't just protect the local population from spawned terrorists, can they!).

In Iraq, the Americans were there for 9 years, and when they left the country, insurgent movements that want to overthrow their loyal regime immediately intensified. The same is true in Libya, but given that the intervention there was very limited, yes, it can be called a short-term success. And people are still coming back from Afghanistan in boxes.

This scales a little more cunningly... In modern conditions, there are not only ordinary bombs, but also atomic ones, as well as those based on nuclear fusion. The United States has already bombed a couple of Japanese cities, right? Now imagine that you have even more powerful or just the same weapon at your disposal, you are not bound by any pacts, and the enemy on the planet has areas with power plants – atomic or whatever, even betarion ones, that is, with a huge energy density and potential harm... Do you think that significant damage will not be done? I doubt it.

And again, several mistakes:
1) First, you are bound by pacts. In the settings, civilized countries prescribe a ban on full-scale bombing and use only limited ones. Of course, there are those who do not do this, but we are talking in general, which means we take into account all possible options – including those who carry out only limited bombing.
2) Given the extremely slow decrease in population with the most aggressive bombing regime, I don't think that nuclear weapons are used on a large scale.