To accomplish this task, you need to follow the instructions for "Acquiring Management Agent Software in Offline Mode", since your EM12c does not have direct internet access. The documentation contains instructions here: http://docs.oracle.com/cd/E24628_01/install.121/e22624/install_agent.htm#EMBSC292
Briefly, you would go to the Self Update page in EM12c and retrieve the URL for the em_catalog.zip file, then download that file from an internet-connected machine and transfer it to your EM12c server. Use the 'emcli' binary in the OMS_HOME/bin to import that file into EM12c with the import_update_catalog verb. After that the Self Update -> Agent Software page should show agents for the additional platforms you need in an "available" status, which you then click on and then hit 'download'. EM12c will give you another URL you can use to download the agent software from another internet-connected machine, along with instructions on how to import the downloaded file to EM12c. Transfer that file to the EM12c server and import it as directed. Then you will have an agent for that operating system available and you can deploy it to your Solaris servers.
You should have an agent directory somewhere on your EM12c server already. I'm not sure where the 188.8.131.52 installer puts it, but you do not need to know that detail in order to deploy new agents to different servers. If you do want to find the agent directory on your EM12c server, go to Setup -> Manage Cloud Control -> Agents, click on the agent whose name matches your EM12c server, and the summary section on the left side of the agent home page should tell you the agent's Oracle home and agent instance directories.
For your direct questions:
1) You can use one operating system for the EM12c machine and a different operating system(s) for your monitored hosts. This is not a problem at all. Some people (including me) would question using Windows at all for any piece of your architecture, but it should work fine.
2) No. You don't need to download another EM12c distribution, only the agent software which you access as described above and in the linked docs.
3) If EM12c is already up and running, you have an agent directory somewhere on that host running EM12c. You do not need to create it manually. See my third paragraph if you'd like to find it. Mine is in a different spot because I have done upgrades from EM12cR1 (184.108.40.206) so I can't say offhand where it gets placed for a new installation of 220.127.116.11.