Putting aside possible security holes in Tor browser, is it risky to make tor browser remember history and/or accept cookies?
The answer to this question depends on how you define "risky."
If you remember history in the Tor browser, the history will only be stored locally. Only people that have access to the computer will be able to see your browsing history.
As far as I can tell, cookies are already enabled in the Tor browser. However, they are deleted when you close the browser. That option cannot be changed in the Tor browser, so your options are either accept cookies until you close the browser, or don't accept cookies in the first place.