Flooring – how better to cut laminate planks — with saws or laminate cutter

Rent a mitre saw. It is the quickest way.

You will very quickly regret attempting to hand saw a hardwood flooring installation. Especially when you need to do a rip cut.

"If hand-sawing is out, are there any miter box attachments for circular saws? Googling has only turned up"

You could use a circular saw and/or jigsaw. If you wanted an inexpensive and quick "plug and play" solution, there are plenty of saw guides you can buy for square or angled cuts.

Note that especially for angled cuts with a saw guide, accurately lining up your cut line with the saw blade can be a challenge. Even more so for someone not experienced doing this.

Ramblings

If I had to install a hardwood floor tomorrow and all of my mitre saws were broken and no rental stores had any available, I would make myself some sort of cutting station. Similar to this design:

In the image above the coloured end piece is cut off exactly in line with the edge of the board above it.

If you stack some wood on the bottom piece the height of a hardwood plank, and leave a slot the exact width of a hardwood plank- You could mark a cut line on the plank, slide it in to the slot, lining it up with the cutting edge and make quick and accurate cuts, similar to speed and accuracy of a mitre saw.

Their install guide says

"Under normal conditions, Harmonics® flooring requires no acclimation period. Normal conditions are defined as a difference of no more than 25°F and 20% relative humidity between the stocking/transportation environment of the flooring and the final installation environment of the flooring. Outside of normal conditions, let the planks acclimate for 48 hours in the unopened packaging at the normal room temperature in the middle of the room where the floor is to be installed.Allow air space between packages"

So looks like you can get installing right away.