In case anyone is still interested, I just called at Guangzhou immigration, they told me as long as the total time is below 24h, entry for 24h is actually possible in URC. They never heard of any other rule, I even specifically asked about the 2h rule. There is an English hotline in URC, but it seems not to be working (busy whenever I try to call). And I totally would ask at VIE airport, but itīs 500km away, so no option sadly.