Q:

Widest tyre you can safely fit on 700c wheel (17mm rim width)

I am riding a no-suspension hybrid with rigid reinforced fork and 700c wheels.
I go on touring trips with very flexible itinerary. I stay on gravel/tarmac/trail 80% of the time but occasionally I like to link parts of the tour by going off-road or on rough trails.
I currently run 700x35C touring tires and I am very pleased with their performance on good surface. However I would like to get some oversized (preferably foldable) tires that I can pack when touring and put on for a day or two during the tour when necessary. Here are some cases from my recent trips when my 35c's felt "not enough" and I wish I had wider tire:

Here are few details:

Rims are WTB 700C
Rim inner width measures just under 17mm
I currently run 700x35C
Disc brakes, so brake assembly will not be a limiting factor
Frame and fork will not be an issue as well (both look like they could accommodate 55+ size and still have plenty of clearance)

Rim inner width:

Ample tire clearance (pictures taken with Schwalbe Marathon Plus 35c tire in place):

Sheldon Brown says I should not go above 37mm...
Yet I know that this bike came from the manufacturer with 47c continentals fitted...
Any advice is appreciated. I would like to hear not only what is OK for this particular case but also what are general considerations. How does one decide when tire is too wide and unsafe.
UPDATE:
Additional problem that I am facing is that it is hard to find wider 700c tires. For gravel / hybrid tires advertised as 700c sizes go from 25mm up to 37mm.
For mountain bikes (29 inch wheel) sizes begin at 2.25 inches which is approx 57mm.
There appears to be a gap between 37mm and 57mm and it is very rear that one come across a tire that falls in that gap category. Or am I not looking for a right thing?

A:

There are loads of 2.1" MTB tyres, and a few 2.0. I've got some WTB nano 2.1s that were on my hardtail when I got it, and I've got a Rapid Rob 2.1 on it now. 
But your current tyres look pretty slick. Putting a dirt touring tyre like a marathon mondial on there would certainly help (and that comes in 35, 40, and 50mm widths). Those are OK on road, though slower than I'd choose for all-tarmac use. I take my tourer off road too (though not laden) and have really noticed the difference between the marathon mondials and the slick marathon supremes I switched to (or indeed the marathon plus on my hybrid), even with both at 35mm.
The footnote to the table you link suggests you can go a fair bit wider - in fact my hardtail has narrower rims than my tourer. You're likely to be limited more by the clearance batten the chainstays/seatstays/forks.
The tyres I mention are just examples, based on what I've used. 

