Dear [Name Removed]
As-salamu alaykum
Please kindly see my responses to your comments in brown below:
"I have a question. How far do you think these "commonly held principles and beliefs" need to go? By that, do you mean the basics such as salat, zakah, hajj, saum and punishments such as lashing etc?"
In my humble view, they would need to go as far as to coalesce around an agreement of the basic tenets of the religion. As only an example, this could be broadly divided into the following areas:
1. Fundamental beliefs / principles [Usuls]
Verse 2:285 would need to be defined and elaborated. For example, what does one mean by angels? The Quran lists this as a fundamental tenet of belief. In other words, there would be no point in asserting a belief in angels, whilst retaining very different perspectives of what the angels actually were. The same would apply to the 'Books' / Scriptures, messengers etc.
2. Basic Practices
As you suggest, this would include a common and defined understanding of 'Salaat', 'Zakaat', 'Hajj', 'Saum' etc. Again, it would be irreconcilable for one to assert that Saum referred to the abstinence of food and drink whilst another believed it simply meant following the Quran or one believed in ritual salaat and another did not. These would amount to irreconcilable fundamental differences of opinion in matters of religion.
3. Legal framework and Jurisprudence
"Or do you think everything in the Qur'an, including the differences of opinion in the understanding of "daraba", polygyny, dress code, "right-hand possessions" etc?"
These could be argued as secondary considerations. Here a difference of opinion would not necessarily be tantamount to a fundamental difference in religion.
"If the latter, then wouldn't that lead to the same problem that we have in traditional Islam today? Of everyone being told to follow the majority consensus?"
Not necessarily, as one would arguably be rid of the huge influence the tomes of secondary source literature places on Islamic thought. Admittedly, there will always be differences. However, a form of consensus should not always be seen in a negative light as the Quran does expect its adherents to come to common ground. How else could a Quran-centric / Quran-based community religion flourish?
Of course, the ability to critique should / will always remain open (outside the fundamentals) unlike much of the areas of traditional Islamic taught which doesn't appear to afford the same privilege or appears to create strong vehement opposition to even the finest of differences.
I hope this helps, God willing.
Regards,
Joseph