Travel into Guangzhou by air and then by direct train to HKG

Hans, if your total layover time in GZ is no more than 24 hours, you are eligible for 24 hr direct transit in there. No need to depart out of Guangzhou Airport for that.

But don't I need to get a confirmed seat of the train or ferry? How practicable is getting a confirmed seat, as I believe those tickets are usually sold at the ticket counter? If not, where and how do I get a confirmed seat of a train or ferry ticket? Thanks Frank

I meant train or ferry ticket from Guangzhou to Hong Kong. Thanks.

Yes, you need to get a confirmed seat of the train or ferry for that. You can try to buy the ticket from the ticket counter, or you can ask a Chinese friend to help you book the ticket on the official website.

And if I want to stay in Guangzhou City for 48h and continue by train to HKG only then, would that be allowed?

No, you are not allowed to take a train to HK.