Opening software/hardware to the community

There seems to be several ways to post hardware & software into the public domain but if you want to post designs and retain some form of control over what happens with them what way is the best.

Copyright - How is this effected by posting stuff for all to see and use?
Prevent/allow commercial use of your designs?
Ensure stuff based on your hardware/software designs have to credit the developer?
Ensure stuff based on your designs has to remain open to the community?
Licensing your designs for commercial use?
Reversing and open licence?

Riva:
Copyright - How is this effected by posting stuff for all to see and use?

In the U.S. the act of creating something automatically gives you copyright over what you created. Posting for all to see has no affect on the copyright; the copyright still belongs to you. From what I’ve read it is very helpful to assert copyright (include a copyright notice in each file). You can also register what you created with the Library of Congress for a modest fee. I have no idea if registering is or is not helpful.

I have also read that, again in the U.S., it is essentially impossible (and potentially opens the author to being sued) for an entity other than the government to release something to the “public domain”.

Ensure stuff based on your hardware/software designs have to credit the developer?

I hate to be a pessimist but, unless you are willing to regularly police the internet-at-large and have an especially nasty lawyer on speed-dial, you will never be able to ensure credit is always given. Just on this forum, there are a handful of examples of code that has clearly been stripped of original credit. I have even seen the Arduino Core stripped of original credit.

Given the breadth of your list (“open” to “commercial”) and the fact that laws vary significantly from country-to-country you will probably have to focus on a subset of jurisdiction(s).