Ok, Now I've seen the video's.
Nice to see a person searching for a better technique while swimming

- it reminds me so much about my own years old search for a reasonable technique.
From the images I get the impression that Silvia is trying to keep her stroke small.
I think you would get more thrust when you have your bottom and back arching up in relation to your head's and arms hight.
Watch your video, and see for yourself you have more propulsion when your back and bottom go up.
To get to this:
- do some warm-ups like back crawl and monofinning on the surface using snorkel and swim-board to support the arms. The goal is to warm up and stretch the upper back and shoulders.
During the dive:
- look a bit more backward instead of down.
- put the heals a bit outside and with strait X legs push your
middle back up - instead of the fin down.
- to help pushing your back up, a light roll forward from the shoulders helps.
- feel comfortable to let you back and bottom make a big amplitude.
Practice this swimming at different speeds.
For training I usually use a lighter weight set-up, allowing me to dive without packing, saving me a lot of time and allowing myself more distance covered.
When I feel like it, I'll do a max, doing that after a no apnea warm up.
Most training I'll use small fins, snorkel and a board practising the movement, strength, endurance, flexibility. At the end I put in my dear monofin and swim some 50m's and some 100m when I have the energy left.
Seeing these video's and writing about it makes me very eager to swim myself!
I hope the lady accompanying your involuntary over night stay was not present in the flesh. You see romantic moments work better with just two people
