While there are a lot of nuances to this I won't get into, the simple answer is that it's hard to control where the virus inserts into the genome (imagine inserting a random page in the middle of a recipe book).
It's also hard to get the virus to target the cells you need to reach, as they often only target one specific cell type.
The virus doesn't need to insert its sequences into the genome. There are different types of virus, and some do (retroviruses) and others don't.
It can be perfectly fine to have the dna floating around in the nucleus or even the cell body to turn it into a protein. Exceptions abound, of course...
Experimenting on otherwise terminated fetuses would be unquestionably unethical because we'd have to let them age a little, which means their brain would develop, they would gain consciousness and would be likely be very messed up, then have to be "put down". That idea probably won't get much support...
We can and do use monkeys, "Planet of the Apes" is based on this. Attempting to cure alzheimers created smart monkeys, then a bunch of unknown stuff happened and they took over the world.
I'm not going to say impossible, but we just don't need them at this point. Plus since a lot of these genetic experiments would be ultimately affecting something in the brain, it might be near pointless. I don't know, I just don't see it being plausible or necessary or allowed anytime soon.
If we can grow headless bodies, we can just use the organs for transplants, so... that'd be much easier.
I don't know, that's kind of a strange question that I don't feel comfortable even considering. Not many scientists would be comfortable experimenting on headless humans either.
Understanding DNA/RNA and simulating it with computers while using animals to verify the algorithms might be more effective.
Basically science is: predict what will happen, try everything, see what happens, study the results, don't blow yourself up or kill people. Mice and other animals are debatable to harm if necessary, greater good I suppose.
Organ transplants would be the biggest benefit, possibly even full head/brain transplants which would cure almost all conditions. Once we have the technology to reconnect nerves at least.
Yes you can't test what happens to brain tissue ethically, but you can test on every other organ and tissue which would be a huge advantage.
Not necessarily. The population of some countries allows their government to kill (selected) people, but not to experiment on them, so the first doesn't necessarily imply the second.
It's also hard to get the virus to target the cells you need to reach, as they often only target one specific cell type.