If I post a message from an ISR does that message get immediately processed by the RTOS and acted upon or does it only get acted upon at the next RTOS tick?

If the ISR activates the scheduler (The method is port specific) then a task waiting on the queue/semaphore the ISR acted on (if it is now the highest priority ready task) will be activated, otherwise it will need to wait for the next time the scheduler is activated, which should be no later than the next tick (assuming it becomes the highest priority waiting task).

If you call a function from an interrupt that causes a task to unblock, and the task that unblocks has a priority above or equal to the currently running task, then the interrupt will return directly to the unblocked task if you tell it to.

Here is a generic example. Note this uses a macro call portEND_SWITCHING_ISR(). In some ports this is called portYIELD_FROM_ISR(), which does the same thing.

void Dummy_IRQHandler(void)
{
long lHigherPriorityTaskWoken = pdFALSE;

    /* Clear the interrupt if necessary. */
    Dummy_ClearITPendingBit();
    
    /* This interrupt does nothing more than demonstrate how to synchronise a
    task with an interrupt.  A semaphore is used for this purpose.  Note
    lHigherPriorityTaskWoken is initialised to zero. */
    xSemaphoreGiveFromISR( xTestSemaphore, &lHigherPriorityTaskWoken );
    
    /* If there was a task that was blocked on the semaphore, and giving the
    semaphore caused the task to unblock, and the unblocked task has a priority
    higher than the current Running state task (the task that this interrupt
    interrupted), then lHigherPriorityTaskWoken will have been set to pdTRUE
    internally within xSemaphoreGiveFromISR().  Passing pdTRUE into the 
    portEND_SWITCHING_ISR() macro will result in a context switch being pended to
    ensure this interrupt returns directly to the unblocked, higher priority, 
    task.  Passing pdFALSE into portEND_SWITCHING_ISR() has no effect. */
    portEND_SWITCHING_ISR( lHigherPriorityTaskWoken );
}

Regards.

Awesome - this is the first time I've needed to use the HigherPriorityTaskWoken flag and had forgotten about this behavior - that's most likely our issue.

If most of our tasks are at the same priority, then will returning TRUE cause a task of the same priority level (assuming it is running) to be immediately preempted?

Many thanks Richard.
Regards
travfrog