Much would depend on an analysis of commute pattern, if there in fact is much, from these communities as well as Carleton Place. If they are to the tech hub (and now NDHQ) in Kanata/west end, it is closer, but I still can't see many people schlepping down from Pembroke/Petawawa on a daily basis.
The Carleton Place Sub was completely torn up in 1985. Given that it has always ended in Carleton Place (intersecting the perpendicular Chalk River Subdivision), it would end up being a rather short line (Carleton Place is only about 55km from Ottawa Station). With the population of Carleton Place, Stittsville and Bridalwood, trains could return to it one day, but I am not holding my breath.
As far as I know, the Beachburg sub is torn up west of the junction with the line to Renfrew.
Yes, they were removed in 2013 and 2014, but the part that connects the Renfrew Sub to Ottawa is still intact. A couple years ago, CN put on its 3 year plan that it intends to discontinue the remaining section of the Beachburg Sub, but I expect there will be a similar deal as the Renfrew Sub whereby the City of Ottawa buys the ROW and the ANR buys the track.
Another complicating feature for use of the Beachburg ROW is it would have to be federally regulated since it hops into Quebec.
Or at least it did until all of the track in Quebec was discontinued. I am not sure how much of a complication that is though. The O-Train's Trillium line runs on federally regulated track (it continued across the Prince of Wales Bridge into Quebec, until the track was torn up without properly applying for discontinuance ).