Power on state of D Type Flip Flop
There isn’t a hold time problem if you add an inverter - in fact this helps hold time, but subtracts from setup time.
No flip-flop is guaranteed to power up in a known state. They all require a set or reset to initialize them. You’ll need to add this, or design your logic so that it doesn't care.
As you said, the ‘79 flip flop could be initialized to a known state by forcing the input and clocking it. This requires an additional gate to the D input, and coordinating that state-forcing with toggling the clock. Probably more complicated than you want.
The ‘175 flip-flop has an asynchronous reset, which works regardless of the clock. You could indeed do some kind of RC circuit to make a reset at power on for a cheap-and-cheerful solution.
Finally, static flip-flops like these have no maximum clock period / minimum frequency. Dynamic flip-flops exist, but not as small ICs like the ones you’re working with here.