Hi Gregr,
before I start to explain I’d like to point out that the efneo wasn’t designed just for Strida - the Polish guys created a drive which should be suitable for ALL bikes.
You overlooked that here are two things to consider; one is the design and the other are the technical specifications of the installed drive.
To comprehend, we need to take a look at the inner gear ratios of the mentioned drives, these are like this:
1st EVO gear: 0,787
2nd EVO gear: 1,0
3rd EVO gear: 1,257
1st efneo gear: 1,0
2nd efneo gear: 1,43
3rd efneo gear: 1,79
(Note that the direct gear, represented by the factor 1,0, is on the EVO drive the middle gear while it is the lowest gear on the efneo.)
So, if we mount a 100 teeth beltwheel, we could achieve these teeth counts:
1st EVO gear: 78,7 (virtual) teeth → lower than single speed ratio
2nd EVO gear: 100 (real) teeth → same ratio than single speed
3rd EVO gear: 125,7 (virtual) teeth → higher than single speed
1st efneo gear: 100 (real) teeth → lowest ratio equal to single speed (too high for a low gear)
2nd efneo gear: 143 (virtual) teeth → a bit higher than 3rd gear on EVO (should be manageable)
3rd efneo gear: 179 (virtual) teeth - > much higher than 3rd gear EVO (too high to pedal)
The inside gear ratios of the efneo drive (just like on Schlumpf/ATS drives) require explicitly the use of a smaller beltwheel, with 80 teeth are the efneo tooth numbers:
1st efneo gear: 1,0 x 80 = 80 (real) teeth → lower than single speed ratio
2nd efneo gear: 1,43 x 80 = 114,4 (virtual) teeth → a bit higher than single speed but doable
3rd efneo gear: 1,79 x 80 = 143,2 (virtual) teeth → a bit higher than 3rd gear EVO
Regarding Gates:
(It’s so boring…what a pity that some companies, like Gates or Ming cycle, obviously really do not like to talk with simple customers…)
The last request, kept very plain, was sent to Gates Aachen already at 14. 05.
They were so far not able to reply… 
Best regards,
Chris