Lavender oil? I know peppermint extract or smoke test. It can be many reasons for bad smell (wax ring, AAV, vents....and the list go on). Do yourself a favor, get a professional plumber to test your sanitary lines and vents and be done with it.
3' extensions raiser plus a pop up sprays= 4' tall (or more) so your AVB or PVB shall be at 5' (12" above highest outlet). I would use a DC or an RPz (install 12" to 36" above ground).
This should help. I'll try to login later to check your test after shut off 2nd valve (I'm in Florida and a bit busy with hurricane). Try the last one to flush any debris, if still dripping then a backflow certified shall be clean/fix it.
Please shut off the water using the shut off 2 in your backflow (#1 shut off is the one on the inlet and the #2 the one on the outlet) and report what happen.
Start simple, check the leak detector dial in your water meter, if spinning then call for leak detector. Also a mechanic stethoscope will help to listen if leak (NOT to pinpoint but confirm).
SureSeal, trap guards are just a temporary solution. I couldn't find the pic in my phone but found where I posted here before http://www.plumbingforums.com/forum/showthread.php?t=8175
Do it right the first time, p-trap and trap primer is the way to go.
You must have at least 4 years of experience as a plumber. This may be proven with wage stubs, tax returns and certifications by other master plumbers. Up to 3 years of that experience may be provided with proof of a qualifying degree. You should also have at least 1 year of experience as a...
If you can't cut behind the trap adapter and use a 45 pvc elbow, then use the tubular or 45 compression elbow in your picture. My diagram it's not the best one but will help you understand.
That's they way I learned or I was told when I started. R-5 in drain (clean out plugs etc) and tape + R-5 if water pressure. I carried in my truck others for different applications/scenarios.
Putty on top (not too much or you will plug the over flow) and pipe dope on the thread like spcwaters mentioned before. -personally, I like to use Rectorseal 5 underneath-