Wow... Wow... Real experienced advices! Thanks!
Anyways, the floor should waterproof good enough since renovation 15 years ago! And, I think your 3rd point perhaps the issue of the current incident!

Ok. The injection, yes I saw that works before in my office! And it seems quite work well ya!

Another consideration, whether those Acrylic Waterproof Liquid products Ok for my leaking issue? I heard many of them out there used it and quiet happy with the result! What do you think?

Lastly, as your experience, that's definitely not the Pipes Cracked (inside wall) ?