Vent on dry return as substitute for main vents

My daughter's house has an oil-fired 1-pipe system in a small two story apartment. There are two mains and they run off into pretty much inaccessible points in the basement, at least in the short term. The mains are both 2" iron and about 15 feet long. Neither have vents. The returns are 1" iron. The two returns are both dry. On each return there are two vents, one is about half way back to the boiler and the second near the boiler. All four are Ventrite #35 but are stuck open. There are six radiators three on each floor. The basement is very warm - no insulation.

The short term plan is to review the radiator EDR and boiler sizing, check for water usage, repair the vent situation, and insulate as much as is accessible. (Sadly she only told me her heating bills were high recently so she's missed most of the winter, but there is next winter.)

For the vents I'd like to put Gorton #2s on the end of the mains but as I said the ends of the mains are pretty much inaccessible and will involve major work to add the taps. For now, my plan is to plug the two vents near the boiler. For the other two, replace each with a Gorton #2. The VentRites are on a vertical tee with a 3" nipple. Will having the Gorton's on the returns as I am proposing cause any problems in the returns, or damage the Gortons being too close to the return pipes?

Thanks,
Harry