The reason why the voltage won't increase and the charger heating up are likely directly connected... I highly doubt that the power delivery systems in that laptop are designed to overvolt, so they're probably straining to provide the additional amperage being drawn from increasing the clock speed. The charger heating up would be a combination of the additional amperage being drawn, and from the laptop trying to up the voltage. It may be that the charger is not providing enough power to up the voltage and the amperage at the same time, so the laptop simply is unable to raise the voltage.
Let's say you have a 90W charger. It provides 18 volts and 5 amps. I made these numbers up, I'm just trying to use friendly numbers. Let's say that you raise the clocks in the laptop, causing it to draw 6A instead of 5. This would mean that the charger would no longer be providing 18V, instead it would be providing 15V because it can only provide 90 watts.
There is step down circuitry in the laptop. The lower the incoming voltage from the charger, the lower the amount of voltage available to the internals of the laptop. Since the laptop needs to convert that 18V down to 12V, 5V and 3.3V for the internals, any drop in voltage from the charger would mean more amperage being pulled through that connector. The laptop is likely hard coded to limit the amperage being drawn from the charger, so that the charger does not literally burst into flames. This means that you likely have hit the limitations of the charger and are not able to increase voltage to the core because there is no more available power.