The weight of the ebike has marginal effect on the efficiency of the battery. Most of the energy goes into acceleration, not into actual maintaining of speed. What an ebike give you is an ability to very quickly accelerate from stationary position. I have ebikes of different weights, from 16 to 27kg and I see no difference in battery consumption. Ultimately, the weight of the bike is marginal compared to my weight (77kg) so a +/- 10kg change in the weight of the bike only translates to just around 10% total weight saving. It's just not worth it.
Some of the things you can have as a result of "f~k weight economy":
- Install a heavy planetary gear hub such as Rohloff or NuVinci
- Install a rack and put a load on it. I use the rack to take my son to school (30kg)
- Use heavy 1.5mm bicycle tubes; this is puncture protection at the cost of extra weight
Honestly, I wouldn't worry about weight unless of course you go for carbon. Carbon doesn't take the stresses so well, not to mention that any direct impact has a chance to just break it. With carbon, perhaps you do need to calculate weight. And I do know that carbon ebikes exist, but they mainly exist because they are commuter folding bikes that you need to be able to pick up. Most ebikes are too heavy to pick up, which is why manufacturers add Walk Assist functionality so you can get them easily up the stairs.
About pedaling the ebike when not on battery power. On Yamaha: yes, doable. On Bosch: no way; the front sprocket is too small to make pedalling comfortable. And if you go for a gear hub, well, that makes things even worse: pedaling a Rohloff without assist is possible, and if you run out of power you can do it for short periods of time, but it's not very efficient and you won't go fast. You should be OK pedaling on a Bafang, but I haven't tried it myself so I cannot say for sure; Yamaha bikes have large front sprockets so it's not such a big deal there.
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