What is your purpose of an initial value and mandatory at once?
There is a deeper problem with the slider definition.
We cannot use the bootstrap-slider with an inital value and no value.
Maybe we should deactivate the at-once usage of initial/default-value and mandatory.
We are open for ideas here.

This issue about appearance at the first place.
i think that 'slider_user_noaction'.$myfname should be saved in survey session or may be in cookies.

Or another way.

Part of slider's HTML code:
<div class="slider-handle min-slider-handle" role="slider" aria-valuemin="0" aria-valuemax="100" aria-valuenow="5" style="left: 5%;" tabindex="0"></div>

Only for mandatory sliders.
Can you change style="left: 5%;" to actual percents instead of change slider value? 50% if "Slider starts at the middle position" is on at least.
5 is magic number from bootstrap-slider.js, string 734: defaultOptions: {value: 5} so sliders always have something as value.

Now I think its bootstrap-slider.js issue: https://github.com/seiyria/bootstrap-slider/issues/614

I tried to insert something into the slider
https://github.com/seiyria/bootstrap-slider/pull/612
But is was declined, so I think we will fork the slider and implement the null value.
When we do that, we can also think of a way to position the slider somewhere without actually setting a value rendering the extra attribute ´slider_user_no_action´ unnessecary.
Would that be a solution?

Yes, with that change you can set slider visible value to center and ignore "Slider starts at the middle position" option for mandatory questions with sliders. Or may be add additional parameter for "Slider initial value" field. And i'm sorry, i'm used limesurvey version with unchanged js 9.1.3 file when i typed (40238) note.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=21765

New version released.