there are three ways of doing this (plus any others I may not have come across). The first is something along Daniel’s lines. It was the first I tried and often worked well but I had problems with a number getting a decent layout – possibly my fault. You said it solved the biggest part of the problem though that suggests there is still something perhaps not ideally functioning?
No.2 is simply using a KSP Kontakt script such as the one from Alex Vincent which you can read about here CSS Control Panel - CSS/CSSS/CSB/CSW legato delay solution + other features (1.8, now with Winds!) | VI-CONTROL This allows single note trills. I don’t know of a way to detach this feature from other aspects of the script which is primarily aimed at implementing the correct delays for different articulations. It may work for you but I found some issues with sticking notes in fast passages and incorrect application at times of the two marcato patches.
No.3 is what I use currently. This to create an extra staff to notate the two notes of the trill which will then be used for playback. Procedure depends on whether you are using solo or section strings.
If solo do the following a) create trill as for other libraries using the correct interval in trill Properties and select the bar b) right click for context menus then staff–> add staff below. c) enter the two notes of the trill on the new staff in the same location as the original trill and finally d) go back to the context menu and select staff → remove staff. The trill notes on the hidden staff are retained and used for playback
If section then a) and b) as before but instead of “add staff below” , choose option “change divisi”. Click on the icon for “add section divisi” and choose 2 under divisions. c) exactly as above d) after doing that you will see a new “divisi” signpost. Simply delete it. The staff will disappear but again the contents are retained.
This all sounds complicated but in fact can be done in a few seconds once familiar. The two methods of “add staff” and “change divisi” are documented in the Help. If it doesn’t work as excepted ensure you have the correct trill interval, ensure that the enharmonic spelling is the same (for the trilled from note) and to be sure, add an “ord/nat” beforehand in case there is another active playback technique needing to be cancelled.