Using a full motion single stud wall mount for the 49” Sony 900f?
Yes, a single stud CAN support your 30 lbs. T.V., if it’s installed PERFECTLY. 

The mount is designed to 1) hold the tv up, and 2) keep the bracket (mount) from rotating off the wall. 

1) The weight of the tv is not significant. A standard bolted connector will easily support 30 lbs. 

2) The stress from a tv rotating off the wall is complicated. The bolt at the top of the connection is in withdrawal. That is to say, the bolt wants to “slip” out of the wood. To resist that force, the grooves between the bolt and the fiber in the wood is the only thing resisting that withdrawal force. 

If the bolt is installed off-center of the stud, it may not develop full resistance. Also, if the bolt is installed too loose, it could loosen over time due to your “often” movement (vibration between the connection and the stud) of the tv. 

Generally, structural design is redundant. That is to say, there’s always a backup. When you fasten to just one stud, it had better be PERFECT.