I won't go into all the possible options here as quite extensively discussed. My main point would be to inject a bit of realism into what you are trying to do here.

Yes, I get the enthusiasm and maximizing the experience and all that, but to me it all comes down whether you are willing to consider a more achievable schedule / itinerary or not?

I'd try to get LX F to SIN and if you can really build in J/F side trip within Asia kudos to you. Even before Covid days, your schedule was well maxed out and in the current situation there are so many unknowns still with HK / Japan and travel to Asia in general that there is bound to be a "spanner" (issue) at some point that could jeopardize even the most carefully crafted alternate itinerary.

The KBP F fares were very well priced, so you are still getting an awesome deal, so I'd not make it too complex under these circumstances. I love a challenge as well, but this would drive me nuts. YMMV. Good luck.