Is it permissible for an orphaned girl to take money from her paternal uncle through investment certificates that the uncle places in her name in the bank, from which she receives a fixed monthly amount, knowing that this money is her deceased father's end-of-service benefits, and that the uncle claims to have fatwas (religious edicts) that permit this? Otherwise, is the uncle obligated to spend on her from his own money, given that he is her guardian?

If you have evidence regarding your father's money, which his family denied, then you have the right to claim it through all legitimate means. As for your uncle's handling of your father's end-of-service gratuity: if the bank is Islamic, there is no harm in benefiting from its profits. However, if it is an interest-based (riba) bank, then it is not permissible to deposit money in it. And your uncle has no right to prevent you from disposing of your money in a permissible way.