Would you mentor other people for free/voluntary donations?
I is hard to give a sufficiently nuanced answer to this question. There are many factors. I will try to give my own perspective. 

First, I give away a lot for free in this forum and in others, both within the SE framework and to the SIGCSE and APCS communities. So, my basic instinct is to be generous with my skills and experience. 

Second, not everyone is as economically advantaged as everyone else. Some people would like an education but various factors, both economic and political, make that very difficult. In the US, the tax system used for support of schools is incredibly unfair to many people. If you live in a rich area, you have good schools. If you live in a poor area your schools will find a very difficult time providing you the education you deserve. 

Third, an educated populace is a good thing. The education available to an individual shouldn't depend on where they live (locally or otherwise), or their particular economic circumstances. To insist that everyone must pay their own way for an education leads to a sub-optimally educated general population, which is a poor path to progress for a nation or the world. 

However, fourth, if something is given to you, you may not appreciate it sufficiently, or work to optimize the potential of the gift. I've retired from academia (several years ago), but still teach Tai Chi. It has always been the practice of Tai Chi masters that students must pay something to obtain instruction. Even if it is just a few eggs or a kilo of grain. And the purpose of payment isn't for the support of the masters, but so that the students will be more likely to apply themselves and take advantage of the instruction. It is, in many ways, a mutual gift system. 

There is a common flaw in free online courses, that students tend to drop out at too-high rates. If they commit nothing, they probably don't feel obligated to commit to the work required to learn. People paying for higher education, often work hard because they are already "investing" a lot of money and want a good return and will therefore "invest" a lot of effort. 

So, if you want to know whether I would, personally, teach for nothing, you'd have to tell me who the students are and what it is that they are willing to "invest" of themselves. If you can find a way to somehow assure that they will do the work necessary so that my efforts aren't just wasted, then I'm willing to listen. But, yes, I'm willing to contribute, provided that I think I can make a difference, somehow. But that is easier in small face-to-face groups than in broadcasting to the web. 

Finally, providing the conditions for true learning online are very difficult. If you read other things I've written here and on academia.se you will understand that I have definite ideas about learning and what it takes (primarily reinforcement and feedback) and both of those are hard to arrange faithfully on the web.