Am I right, that Trllls, Tremoli, Glissandi etc. are still not (yet) “played” or reproduced in the Mididata of a certain score written in Dorico 2.
Does the xml-file reproduce them in the mididata?
If I am right and Dorico does not reproduce Trllls, Tremoli, Glissandi etc. in the Midi or played xml-data: Are there plans to do so in future?
Tremolo is played, but neither trills nor glissandi are played. Yes, we do plan to play them all back in future.
Are Trills, Ttremoli, Glissandi etc. currently imported in xml-Format when I load an XML produced by a Scan-Software, which reproduces them in the mididata?
Yes, all of those notations are imported into Dorico via MusicXML.
Tremolo playback should work for all tremolos that last the duration of the note to which they’re applied, is this correct?
For me, Dorico is playing only unmeasured tremolos. e.g. The following plays as unslashed eighth notes; I hear the intended effect only if I apply three or more slashes.
Milo, see the options on the Timing page of Playback Options.
Actually, I did have a look there, Daniel. I found no setting that would make a single stem-slashed eighth note play as two sixteenth notes.
I tried adding a single slash to the stem of a quarter note, as well, expecting to hear two eighth notes. Nope, it plays the quarter note just the same.
I tried playing the notes – which are for viola – at a greatly reduced tempo (q=20). Each note is audibly broken in two at that speed, although each repetition plays back significantly softer and at a lesser dynamic.
If Dorico is actually reproducing an alternating down-bow/up-bow effect on the tremolo notes, then that is super cool. 
It might also explain why the notation isn’t so discernible at faster tempos.
Try writing your single-stroke tremolo on a keyboard instrument where the attack is clear and you’ll hear what effect changing the options there really have. I think the real problem here is that the attack on the string samples in HSO isn’t fast enough for this kind of fast writing, and unfortunately there’s not a great deal of scope for changing this with the controls provided for the HSO sounds in HSSE.
I’m not using the HSO samples at all, actually. I’m using EWQLSO Platinum, and when I play the same music in Sibelius (using the same samples, obviously), the measured tremolos are clear.
I’m simply writing out the notes in Dorico. That gets me the expected aural result.