Peter, the answer to your first question is yes. The answer to your second question is also yes. There are two possible solutions to this problem. The first is to extend the roof vent pipes higher above the roof. If you still have sewer gasses carried into your yard by down drafts, then the second solution is to place Activated Carbon Roof Vent Filters on all of the roof vents.
I have had good results with “Sweet Air” roof vent filters in solving this same problem with my septic system clients in the past. You can view the filters on a web site, and order the filters from Richard Septic Systems, Inc. of Torrington, Connecticut. Jim Richard’s web site address is;
http://www.geocities.com/richardseptic/accessories.html
The filters are “guaranteed” for 90 days, but they are effective for several years. The activated carbon can be replaced when it loses its effectiveness. JWA