You're little daughter is adorable! And already playing the piano too! Reminds me of our own daughter at that age, but who is now 33, although she never became a musician. Enjoy your children when they're very young because they grow up far too quickly.
Regarding that measure in Bortkiewicz's 40/6: In the RH you have a short scalar figure G#, F#, E, all the notes of which are double-stemmed and marked tenuto, with that dotted E also having a dotted line to the D#, also double-stemmed and marked tenuto in the LH, indicating voice leading between the E and D# as you correctly deciphered.
Not only do scales reinforce the intervallic relationships between notes in a key signature, but also within a composition they can actually take on a momentary melodic role as you know. For that reason, scalar passages are of special significance to listeners, thus need to be appropriately etched by the pianist. Such is the case here.
On fingering, when I suggested the 3 on the F#, it was because if I held the E that concerns you with the thumb and then played the F# and the following melodic notes in the measure also with the 5th finger, I could do it only with discomfort which is to be avoided. Using the 3 on the F# assures comfort in playing but sacrifices retaining the E. So the descending scale you wish to bring out as mentioned above, has to be accomplished differently. The way to do it is to etch the G#, F# and E, not only sustaining each one for full value, but also giving them a slight accent as well for clarity. Note that the D# in the LH, serves dual purposes: 1) the continuation of a melodic line through voice leading and 2) a harmonic counterpoint for the A above it in the treble staff. Evidently, Bortkiewicz gives priority to the voice leading aspect, as melody usually trumps harmony. The best way to get it just right is to lock into your ear the exact dynamic produced by the preceding E in the RH leading to the D# in the LH. When you play the D#, replicate that exact same dynamic, thereby etching it the identical same way.
Letting go of the E is one of those compromises in pianism that we are often forced into because of competing demands within a measure--in this case, a physical impossibility (or at the least, a very uncomfortable possibility for the hand) versus a need for voice leading. Of course, Rachmaninoff which his huge hands could do this easily!
Speaking of composers, they often think orchestrally rather than pianistically which can lead to some of these compromises. Anyway, in these situations, be they ties, double-stemmed tenuto notes, a long note value, or whatever, try to hang onto it to the extent practical given the limitations--in this case for one beat only which is better than nothing--and if the pedal can assist further, so much the better.
I hope this helps.