Do they allow 24-TWOV from the Philippines with a layover of 16-18 hours?

The first and the most important detail is that your itinerary, both ways, is definitely eligible for 24 hour visa free transit and you do not need any kind of visa.

Upon landing in Guangzhou the first thing you should do is to approach the 24/144 hour visa free transit counter where you will apply for a temporary entry permit in order to be allowed to exit the airport. This application is free of charge and here you will present your flight tickets and your accommodation booking confirmation. After a short check your temporary entry permit will be granted because you have a long layover. When your temporary entry permit is granted you will cross the immigration and after that you can enter the hotel and if you want you are free to enjoy your time in the city as well.

You will just repeat the same thing on your travel back from Jakarta to Manila. Have a nice journey.

Just a follow up question. Under this circumstance, can I also travel up to Shenzhen from Guangzhou?

Yes, you can travel up to Shenzhen by any mean of transport you choose, but you should have in mind to return back to Guangzhou airport on time in order to catch your onward flight.