My response to the money aspect of this it's just like any other business: money needs to be invested, and then a return will be realized. Resource extraction (i.e, asteroid mining) is one obvious example.
The human compatibility issues with microgravity are well known, as is the solution, which has even been proposed by NASA: centripetal force to create 1G for the astronauts.
As far the the radiation goes, we do indeed know exactly what kinds of radiation they would encounter. And the easiest way to shield humans from it in space is lots of water, or metal. We know this from extensive real work done on earth re: nuclear power plants.
The real issue is money, not technical feasibility. Once the dough rolls in from asteroid mining, it bootstraps the financing issue and pays for itself many times over.
Asteroid mining is one thing. Exploring the nearest star system is science expedition where the payback is in societal scientific knowledge and subsidizing technology development that is then made available here for various things (eg a lot of the space exploration tech in the 60s made its way into consumer tech)
https://www.nasa.gov/humans-in-space/the-human-body-in-space...
NASA seems less sure than you do. And considering we have to get to the asteroids before we even start to think about mining them, talking about the money from asteroid mining is putting the cart before the horse.