Thank you for your suggestion. I have raised your request to our Development team but we cannot assure a time frame when it will be put into effect. Please feel free to contact us again if you need any assistance and we will be happy to provide further help.
For merge: you can select a text (which is near to your excerpt's location in the PDF), and drag it into the excerpt (in the PDF, "severely disabled by" in your screenshot for instance), then it will merge.
For comment: Please forgive us that we have a lot of work to do at this moment and this is not a very high priority task which we have time to consider.
In some ways comment and merge are very similar, so I guess you could use merge instead? Very sorry for the inconvenience.
Also, we are now open for third-party plugin developments, if you are interested. Hopefully some people will realise your idea one day, and we are very happy to look forward to it.