I had some schwalbe marathon's and while the tyre tread area was thick the sidewalls were quite thin and I had problems with the tyres as the sidewalls perished before the main tread that you would expect. Some of the generic cheap tyres have quite thick sidewalls. I'm thinking perhaps the thin sidewalls aid tyre rim compatibility. The chart I linked to is a definitely a general good guide to compatibility and safe use.
I've been looking to answer the same question FunkiestMonkey and so long as you have clearance at the forks, no mudguards to get in the way you can use a tyre as wide as you like, e.g. this thread where someone uses 2.25" tyres on a 14mm inner diameter rim The greater the mismatch between rims and wheel the greater the chance of you attempting a turn and having your tyre make it while you continue onward without a front tyre. This also says nothing of rim and bike strength, as bonzobanana says if you want to land a 2 metre drop on a road bike with no suspension but a fat tyre you probably won't have a bike afterwards.
Those are road tyres specifically designed for wetter roads, not off road tyres.
29" and 700c (622) are both the same size (also called 28" in Germany etc) wheel but the size of your rims dictates what size tyres you can fit on plus the clearance of your bike itself both front and back. Hybrids tend to have good tyre clearance where as road bikes are less generous. Scroll to the bottom of this page to see a chart for tyre size vs rim size and compatibility. You'll need to convert the top horizontal line of sizes to imperial possibly. 50mm being about 2" for example. The chart is slightly cautious if you are one size above or below you might get away with it. http://www.sheldonbrown.com/tire-sizing.html As for the compatibility of your bike for going off road it depends on its it design and how strong it is. For light trails no problem but if you are going to be really giving the bike some abuse fully off road over uneven ground you need to think about its specification. Some hybrids are purely road bikes with a more upright riding style using flat handlebars with fairly weak wheels and designed to be light and fast commuters. Those type of bikes would not be ideal off road for sure. The Carrera Subway is the complete opposite with regard hybrids, apart from lacking suspension its a full on mountain bike and pretty capable off road.
It's okay...I may found it easier than I previously found it.... http://www.halfords.com/cycling/bike-parts/bike-tyres/schwalbe-land-cruiser-tyre-700x40c