1-If you train with a 10RM you will have a higher 10RM,training your 1-3 RM will also increase your 10RM but not as efficiently.
2-Again if the set is to/around failure, all motor units are recruited.
3-Actually you will build a higher tolerance working with higher rep ranges simply because you are able to train with more volume.
4-An increase in 10RM is a good indicator for your progress,maybe not for your 1RM,It all comes down to what your goals are.
With that said, bodybuilders are the strongest in moderate rep ranges but get blown out when it comes down to 1 rep maxes when compared to powerlifters. Bodybuilders have bigger muscles, so its evident that you need to work in the moderate rep range to induce optimal hypertrophy.
There are dozens of other reasons (force/velocity curves, specificity of fitness gain, intra/inter muscular coordination) that 1-3 rep range is needed if for some reason you disagree with my points.
"Do not disturb a formula that is working well. The aim is to find the optimum intensity, volume and frequency of training that enables YOU to consistently add poundage for a given exercise, and with wich produces growth."