It was the Imperial Senate, but it was still an essential organ in the functioning of the Empire at that point and not something that could be ignored.
We had yet to reach the point where Tarkin could declare that “fear of this battle station” would keep systems in line. They were still relying on the Senate to help administrate the Empire and maintain functional control. Moreover, Alderaan wasn’t some Outer Rim backwater like Tatooine. It was a core planet with considerable influence that other systems looked to, and Bail Organa had similar influence within the Senate. This was still several years before even the likes of Mon Mothma could be declared a traitor and chased out of the Senate by the Empire. If they pissed off Alderaan enough to create direct rebellion, dozens of other systems would follow suit.
Moreover, ignoring the specifics in this case, this also steps upon a pet peeve of mine. Just because a character doesn’t follow through with some alternative “better” plan doesn’t mean it’s some sort of plot hole. Characters are rarely intended to be perfect, and are humans (or occasionally droids or aliens in the case of Star Wars) with all the flaws of humans. Sometimes they simply don’t think of all the alternative options out there, or have various illogical reasons not to pursue an alternative plan. That doesn’t mean the plot is lazy or has gaps, only that a character devised a suboptimal plan.
This is not the only case where the show exhibits plotholes or lazy writing.
I respect your attempt to cite lore to fix up gaping plot holes. However, I'd say one major problem with this show that you've kind of maybe touched on is that it simply shouldn't exist. It's hamfistedly retconned into the canon and they have to try and bend over backwards numerous ways to try and explain it. And they still fail most of the time.
Even if what you're saying is true (and it is to a point, the senate still did involve itself in governance), the empire was never above removing obvious political opponents and installing sympathizers. As such, people like Bail and Mon Mothma had to engage in skullduggery and subterfuge to never show obvious rebel ties.
However, because of this show, the Inquisitors and therefore the empire know that Bail is in contact with anti-empire elements. Palpatine would have to move against him in this case unless he's an idiot.
The show also opens up other plotholes they have to try to explain away.
-Palpatine has to give Vader a therapy session to explain why he doesn't go after Kenobi for the next 10 years, but then why does Tarkin think Obi Wan might be dead in ANH?
-They try to explain away Obi Wan knowing Leia in their last scene together, but then why wouldn't Leia show some emotion when Obi Wan dies? She's suddenly kind of a jerk.
-Why does Obi Wan let a genocidal mass murderer with godlike powers live after seeing the damage he's caused in intervening years? This is especially true since he believes Vader is no longer Anakin, as per the ending.
-Want laziness? Why does Obi Wan let his powers slide when his only mission in life is to protect Luke from the Sith? Lazy writing, that's why. They need to create an artificial arc for him so that's how they do it.
And these are just overarching plotholes in canon. The show is perfectly adept at creating plotholes within itself as well, some of which I've mentioned in my previous posts.
Personally, it's probably the worst thing I've ever seen.
