I can certainly understand what you are saying here, however, I would like to suggest that it is POSSIBLE to do this however the probability for success is probably very small for the average practitioner. As it has been said on this board by many different people, there are not too many ways to punch and kick someone. Most martial arts have been influenced by other martial arts throughout history. There is a very high probability that many people have tried to combine elements from one style or system with another with varying degrees of success. In the instances where people were successful in incorporating these elements, new styles were born. If there were or are enough people that recognize some value to what they are being taught, then the style will be perpetuated.
In summary, I think you are probably correct when you say that mixing material from Chinese methods like this is going to get you garbage but I would add for the typical or average student. I wouldn't rule out the possibility that there is or will be someone that will one day be successful at combining these elements to for a new system that will be able to operate with these different foundations perhaps not all at once but in some fashion that will give birth to a new Mantis, Snake, Hung Gar system.
Perhaps that is the optimist in me to think that everything is possible given the proper time, energy and effort mixed with the ability of humans to continuously evolve. Before man landed on the moon, how many people could think of a way to accomplish that ?