But the second case I guess now is not allowed, as before you can go into the city for a few hours. So would the Singapore DoH refuse entry if a passenger went into a Chinese city during a transit?
I'd say as soon as you've been landside and got a stamp in your passport, entry will be refused. But these are only my own speculations. A call at Singapore's MoH might be the only option for clearing it out - there's a hotline number mentioned here:
https://www.moh.gov.sg/2019-ncov-wuhan
But then, of course, it might change until you arrive there.Personally I don't think that Corona will be too much of a problem in April.