Hi Agnes,
It sounds like you've covered all bases. None of my recent work has included multiple results, but I have done it before in testing, when it appeared to work. I don't remember if this was back with Studio 2011 or 2014. Could it have something to do with the way you are constructing the return results in the SearchSegment method? My approach would be to step through the code and check the values being passed back to Studio. I imagine you have probably already done that to some extent, but perhaps you can find if there is a bug in your code somewhere in that part of the method.

In any case, my plan is in fact to move away from implementing translation providers altogether and instead do everything from a custom view part as part of the integration API. I just found out that you can modify the current segment in the document from a custom editor view pane, including the match score, segment confirmation status, origin, text, etc. The advantage is there would be much more control over the display in the view part than you get with the built-in translation provider interface. I will probably post back to the community with updates on this for anyone who is interested.

Not sure how you've implemented this but you can have a look here. This plugin is a translation memory provider that is calling all configure MT providers and then merge the results from all of them.