Well, I've ran with a good 20 pounds in my basket. The weight is born by the aluminum angle at the bottom that's clamped to the seat stay; the only purpose for the upper mount is to stabilize the basket - it handles some 'outward' load, but not a lot. If you have a 20 pound load, with the center of mass at the center of the basket, outward 'pull' at the top will be less than 4 pounds. The lower the mass, the less the outward force. And, the closer to the inside, the less the outward pull. Even so, the dead load outward pull at the top could never be more than about 11 pounds for a 20 pound load, even if the load were centered at the top outside of the basket.
As far as wheel loading goes, I'm an ex-college defensive tackle, and when I started riding, I tipped the scales at 360 pounds. (I'm down almost 60 pounds now!) The bike/motor weighs in at about 50 pounds, so, I was looking at a 410 pound dead load. If you estimate a 60-40 split between rear and front wheels, we're talking about 250 pounds on the rear wheel and 150 on the front. Now, I won't say that this is a safe limit. But, I did run at that weight for over two years. I also was always on the lookout for potholes, avoided them when I could, and went over them 'standing' with bent knees when I couldn't, to reduce the stress.
The first bike was a cheap wally-world cruiser, and after about a year, the rear wheel popped a couple of spokes, and the bearings were starting to go, so I went with a better bike (Trek Navigator,) & have had no further problems. Plus, I love that front suspension.
One other factor that helped; the first 'extra' I bought was a suspension seat-post. This helps a LOT. Not only does it help with rider fatigue, and to reduce both sore backside AND back, but it helps to reduce the dynamic load effects of rider weight on the wheels & bearings. (It spreads the 'shock' out over the time it takes to compress the seat-post, instead of allowing it all to 'hit' at once.) Of course, you've already got a full suspension bike, so that point is moot in your case, but, if anyone has a 'hard-tail' bike, a suspension seat-post can help a lot.