Boroughmuir led in 18 minutes when Chris Laidlaw kicked a penalty for the first of his 19 points, then surged ahead when Grant McConnell broke for a try, which Laidlaw converted for a 10-0 advantage.
The game blew over in injury time. Craig Keddie was sent to the sin-bin by referee Finlay Brown before Alex Cox was also yellow-carded to leave the visitors with 13 players.
Selkirk took advantage when Jordan McGowan crashed over to cut the deficit to 5-10 at the interval.
Despite being two men short, Boroughmuir came out all guns blazing after the interval. Keddie returned in time for Laidlaw to kick another penalty, and it was game over as Otalea raced in for a try converted by Laidlaw.
Laidlaw scored and converted himself before Jake Kerr’s try added the bonus point and Laidlaw converted. Ewan MacDougall scored a consolation for Selkirk and Rory Banks converted.