Thanks, that sounds like a doable approach.

The plumber came yesterday night and sealed the rear of the toilet with bitumen/ asphalt - so far it seems to work, but it is also a little too early to tell since the sewer smell might be hidden behind the asphalt smell right now.

I asked about the vent pipe - he said that pipe is there to allow air to enter the pipes, not to leave the pipes. And since there are no issues with anything being sucked dry, it seems to work fine.

Will see if it the sealing already works, and if not, the smoke test sounds good.