I would like my home page to redirect to a page in a folder. What would be the best way to proceed without hurting SEO or other functionnalities? The page I want as a home page is in a folder and has to stay there.
No matter how you do this, it will hurt your SEO. Google indexes websites by reading their hierarchy, which your homepage is at the top of.
Your homepage should never be in a folder it should be the root domain (domain.com/)
If you absolutely HAVE to do this, you can use a meta refresh in your header:
<meta http-equiv="refresh" content="0;url=http://domain.com/folder/homepage" />
I would strongly advise against this, however.