Degrowth is a noble ideal to strive for, and it would certainly mitigate a lot of our current problems if implemented. However, I fear that it is an ideal that can be adopted by the few but not the many. Growth, progress and personal ambition are inherent human traits - it may not be the case for all people, but it is certainly evident in today’s society and many societies that have come before. In my opinion, we need solutions and frameworks that most (if not all) personalities can exist within. I worry degrowth is wishful thinking, and would love to hear your thoughts.
All of that said - I believe it is a very worthwhile thought exercise and even if all degrowth principles cannot be implemented, some can and that is what matters.
For your first point, yes, I doubt such a system could coexist alongside a more powerful capitalist system.
Second point, I think if you own all the wealth but are socially ostracized, what’s the point of having all the wealth? A caveat to that is most of the wealth in a potlatch society was renewable and not very easy to hoard. Their primary food source was salmon, which can be preserved but not as easily or for as long as grain. In our society, it seems you could hoard a lot and that gives you power.