Plumbing Questions? Ask a Plumbing Expert.
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You're describing a blocked vent issue. The distinction you're making doesn't really matter. You need to get the plumber back to located the source of the vent blockage.
The test is the symptoms you're describing. A water jet might work but only if the water has no where to go except through the blockage.
You need an experienced plumber who can narrow down if not pinpoint the likely location of the blockage before you try water jetting. The water jet can be effective but only if it's inserted at a point where it's likely to take out the blockage and not just run harmlessly down the drain.
Just jetting from the main stack on the roof is at best a roll of the dice unless they have some reason/evidence that the main stack is blocked (possible but not very likely). There isn't some formula I can give you for finding a plumber with the requisite experience. Word of mouth is your best bet or try calling a local supply house and ask who are the plumbers that have been around the longest.
There really isn't much more I can offer from here. There's no need to rip walls apart at this stage. One other possibility for finding a plumber is to try HomeAdvisor.com. They are a free referral service that vets all their contractors. Each plumber has his own profile page so you can check them out. There's even real feedback on the site.