Hello Pavel Tsvetkov 

Unknown said:

1. Situation 1

Translated files are in signed-off mode (all three stages done by me) and I send these .sdlxliff files for further review. The reviewer (an unknown linguist) completes the review in another CAT tool and returns the .sdlxliff files with no track changes back to me. When I open the files, they look exactly the same to the naked eye, but obviously some changes have been implemented.

QUESTION: How can I see what changes have been made? I have tried the app sdlxliffCompare, but it is not what I am looking for, as it creates a report, and I would rather work directly in Trados on the files, not having to revert back and forth between the files and the report.

The only way you can do this in Studio is to compare the SDLXLIFF Compare report so you see the changed segments and then do what's required in the SDLXLIFF in Studio.

Unknown said:

Now, I have tried the Retrofit option, which, for one reason or another, did not work and also the option Apply PerfectMatch is grayed out (not sure why), so what can I do?

What did you try to retrofit?  The reviewer worked on the SDLXLIFF files according to your post so there was no file to retrofit.  That features works with retrofitting the reviewed target file, not the bilingual file.  Perfect Match is a feature only available with the Professional licence.

Unknown said:

2. Situation 2

I have proofed a file for another project and have sent back to the editor an .sdlxliff file with tracked changes and comments (in signed-off mode). Now, the editor has returned to me the .sdlxliff file in question with some of the changes accepted, others not and with further comments. How can I import this newly received file into my project: should I change the name, use a special workflow, etc? What is the best option?

Take a copy of the SDLXLIFF file in your target language folder and then copy the reviewed one you received into the target language folder to replace the original.  When you open your project you'll see the updated one.