You mean, you'd blockade a planet that chose to secede from the Republic? What if the Sith Brotherhood wants you to blockade it, as that makes for a convenient fleet battle.
And what about Onderon.. will you abandon the defense of Onderon to blockade the largely autonimous Zeltros, just to send a message that the Republic doesn't allow planets to surrender?
The US government was not in the wrong to fight the seditious of the CSA. So I don't see why the Republic would be to do the same for Zeltros. Especially if the public does not support the surrender.
From what I understand, the GAR is full of Planetary Defense Forces, who presumably have fleets to defend against pirates and such. I think you underestimate the manpower of the GAR. Or I'm overestimating. Wouldn't surprise me.
Ehm, well, although I'm not an historian I don't think they were entirely in the right either. That said, our Republic is based around trade and that there was until recently no unified army shows that planets have a large amount of autonomy, for example, unlike the States of the USA, a planet should be able secede without repercussions (and even hold a seat in the senate under certain circumstance).
To blockade a planet that does secede is a hostile act that won't sit well with other member-planets, threatening the unity of the Republic as a whole. While you could certainly do it, there's nothing preventing a bully with an army to use said army, the finer politics would advise against it.
Itdepends on what the pub constitution say. Also, seeing as the Republic is at war, and Zeltros being a military objective, people really won't have any room to stand. Not even going into how this could easily turn into a Stalingrad for theSith
As the Republic and Jedi can just prevent any Sith reinforcements easily while bringing in supplies from multiple lines. Unless the Sith make a mad rush through Republic space, and not get bogged down. All while Zeltros is being starved out.
The Republic, in practice, might be more similar to the European Union than historic RL states. As you can see in the faction write-up it's not uncommon that the institution of the Republic is called into question, which would happen if it takes on a more militaristic and aggressive role than its members agreed to.
On another note: The Sith Brotherhood doesn't care about Zeltros. It'll take Onderon, and if that planet doesn't surrender, too, will murder its entire populace. Blockading the planet, therefore, is a waste of resources.
The Republic is at war and most senators seem perfectly willing to protect the Republic. I'm pretty sure they will be fine with legitimate military actions, seeing as they are planning to attack Ziost. Also, if the Sith don't take Zeltros...the Republic can just take it back.