This sounds like an error in the port layer, possibly in the code to setup and start the tic timer. I remember a similar error in the Xylinx Zync port where that code reset the NVIC, so any previous setup was lost.
It might help if you indicated what port you are using.
Do you mean they don’t get enabled, or they do get enabled but are masked? FreeRTOS will leave interrupts masked between calls to the FreeRTOS API and the scheduler being started to ensure interrupts don’t attempt a context switch before the scheduler starts.
If the default handle executes then either you have not installed the interrupt handler in the vector table, or you have changed the vector base address since install the interrupt vector.
As per #1 - FreeRTOS does that already - at the processor level rather than on an interrupt by interrupt level though.
You don’t say when you want to call it - but if it is before the scheduler starts then it is probably ok provided you don’t use FreeRTOS objects that may attempt to use the scheduler, and don’t try to perform a context switch.