How strong is aircraft-grade spruce? I have been trying to find some real numbers on how strong a single solid piece of aircraft graded spruce wood timber is, but haven't had much luck and the numbers that I have found so far were either not for aircraft graded spruce or the sources weren't credible enough or simply the numbers were conflicting with what else I had found previously.

My question therefore is: How much of an evenly distributed weight a piece of aircraft graded spruce can take e.g. as a wing spar or struts before structural failure?

The length and thickness of the timber doesn't matter as long as the spar and/or strut went under the load test alone. The reason that I'm saying this is that I can find numbers for aeroplanes that have spruce spars for example, and based on the maximum G, weight of the aeroplane, etc. I can have a rough guess for how much load the entire wing is certified for, but an entire wing is made of other components that will have an effect on the results of those tests, which means those maximum numbers may not necessarily be the limits of the spruce spar itself e.g. the wing ribs or wing surface might have a lower tolerance, therefore reducing the performance numbers in the certificate.
My recollection of the Modulus of Rupture is about 6000 psi for aircraft graded sitka, based on the maximum allowed grain runout, and a maximum of around 10000 for straight grain.  But you should be able to find everything you need in the old Forest Products Laboratory study at this google search link.

The key weakness in spruce (and wood in general) used for wing spars I've learned over the years is the propensity for dormant failures in compression, typically following a wing tip strike on a strut braced airplane, that places massive compression stress in the wood above the strut attach bolt. 

It collapses the cell walls in that area, evidenced by only a tiny line across the grain, that may or may not even be inspectable, and which drops the wood's tensile strength in the affected area to a fraction of normal.  

Dormant, because you don't find out about the problem until some day when the wing is loaded inverted for some reason, so that the failed section is loaded in tension, and oops there it goes.  For this reason I advise people to stay far away from an airplane with wood spars that's ever had a wingtip repair.