About Control Unit in CPU and Clock Cycle
1.) (Almost) every component of the CPU needs to be synchronized by the clock, which means they all do their work only if the clock is high (1); never when it's low (0) (or alternatively only during rising edge). This could for example be implemented using an AND-gate, with the input to the component as first and the clock signal as second input. So the component will only get its new input if the clock is currently high, else it does nothing. And because the clock speed is set above the longest time any signal needs to propagate through any circuit in the CPU, this system is preventing signals from arriving before other signals are ready and thus keeps everything safe and synchronized. The control signals can be set at any time, but they will only be accepted during high clock.

2.) This is no loop, since the PC is actually a register which is egde triggered. This means, like mentioned above, that it is changing its state only one time per cycle for a short period of time. At an earlier time, the last output from ALU (PC + 1) is just waiting at the input of the PC. Then, at the next cycle, the PC takes its input. This is when PC = PC + 1 actually happens.