I am in agreement with Simon and Wilkar. While it may not be common knowledge, a trading partner is probably going to notice sometimes amiss when all communications from their partner suddenly stopped; btw, jamming is incredibly easy to detect as well.
Agamar is in the Lahara Sector. The Lahara Sector contains 245 civilized worlds according to Wookiepedia. How is anyone not going to notice that? Agamar is in the middle of the Sector according to the map.
The idea is to stave the whole area into submisison since it's a major breadbasket planet.
It's only for a few days. It's not so much of uctting off communications as it is cutting off instant communications. The whole jamming thing will be lifted before any sort of messenger ship can reach a more populated world.
A note, subspace comm has limited range in Canon and TRF stuff (in case you dno't know), so don't feed me anything about that.
People are going to realize it within at the very most several hours after the attack; every civilized sector of space has long-distance subspace transcievers that reroute information between them; it's not instanteous like the holonet, but messages go back and forth between planets within hours of them being broadcasted. If suddenly all of the planets start recieving faint jammings emissions, people are going to realize that.
Lahara Sector from Wookiepedia
Lah
Notice that most of the 245 planets are agricultural worlds. Unless you're intending to take the greater half of 245 worlds, it might be somewhat impractical to starve the surrounding area.
In any case, I don't think this worth a lot of arguing...
EDIT (forgot your last point):
From EGWT:
"Many planetary governments, large corporations, and wealth individuals maintain private subspace transcievers. Most planets are integrated into local subspace networks that use subspace transcievers aboard deep-space stellites to link dozens of worlds in an instantaneous and continuous flow of data."