Best answer I can give is "maybe". If your plumbing is designed properly the toilets should provide adequate flow. But in some older homes you may have some issues. Double flushing for solids is not too unusual for those toilets.

I have seeveral Kohler HET toilets that use 1.25 g/f and have no issue. If the pipe fall is correct it should be fine but tree roots , belly or cracks can screw things up quick

Yes if your building drain follows 1/4" per foot drop code. If you have too much fall, water would be ahead of your solids.

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Does a 1.6 gal flush push the waste out far enough to fall into the septic?

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If by "push" you meaning does it pressure the drainage line up the answer is NO it doesn't. Once the flush siphons out gravity takes over and the discharge follows the 1/4 inch to the foot slope all the way to the street main or septic tank. Cheers, Tom