I know this post is old but you can actually Edit while ingest in Final cut pro x.
You only need a Decklink card, and then you can capture Video using BlackMagic media express software. Use Apple Proress as video codec. Then inport the video to your timeline and start editing. You can refresh the video by switching to mediaexpress software and the back to FCPX (this is the fastest way i think).
I use decklink ultrastudio mini (thunderbolt) to ingest to my macbook pro 2017 model and i can confirm you that under OSX Mojave and FCPX 10.4.2 you can edit while ingest whithout any special software.
Also i noticed that waveform redraw it is much much faster than before.