how to clear an UART interrupt flag manually?
When any interrupt routine is executed then I bit of SREG is cleared to avoid next interrupt execution. The bit is reset by RETI. I.e. you can do it also manually SBI SREG,I  (or via uint8_t sreg = SREG; sreg |= _BV(I); SREG = sreg;).

But also USART_RXC_vect may be called again! I don't like any _delay() in interrupt routines as it must be as short as possible. You should recode it.