I have an Indian Railways ticket, the current PNR status is RAC 9. Does this mean that I'm legally allowed to travel without a sleeper berth, and will be allotted a seat only?
If your ticked status is RAC (reservation against cancellation) then you should board in the train. It doesn't matter your RAC reference number is 1 or 2 or 56, you are allowed in the train and they will give you a seat.
Usually they divide a single Side Lower berth into two Seat and allot them into two RAC candidate.
You can check the RAC status and your allotted seat number just before the train departure time from IRCTC official website. Probably they send SMS notification to the passenger once they allot a seat to a RAC passenger.
Click in this link and search for Can I board the train with an RAC?, you will get all your answer about RAC.
Some typical mentality people with confirmed ticket believe that RAC is same as Waiting List, they have no knowledge about this and during the journey they can start arguing with you. They can tell you that you are the illegal passenger, but don't believe them. Just call the railway officials on the train and put your complain against them in such cases.
Yes you are allowed to.
Once you are onboard, a ticket officer will walk through your compartment, and give him your ticket. He will assign you a birth right away or come back again er a few minutes.
Note that there is a chance you wouldn't get a sleep berth at all on some cases. I would say RAC 1-5 will have a better chance, but the rest is usually up to how many people were not present on the train.
I always find seat61 very helpful when it comes to trains. You can read more here: http://www.seat61.com/India.htm#book - when in India