Below is the tool I used to make the all-in-one .srt file. If you feel my timing is not appropriate in all-in-one .srt file and you want to adjust it, use this tool at your own risk.
How to use: (1) Input the original .srt file in "Source" area. (2) Input the time shift in "shift:" box. (3) Push "Adjust" button. Then the result is returned in "Result" area and message is displayed in the middle area.
I'm sorry but I don't know the exact format of .srt file. In this tool: (1) Only the proper lines in the form of "00:00:00,000 --> 00:00:00,000" with 29 characters are treated as timecode. (2) The number just before the timecode line is replaced with the new subtitle number. If there's no number, it is just inserted. (3) Other lines are just copied. (4) If timecodes are disordered, overlapped, or less than zero as the result of shifting, warning(s) is/are displayed.
I used this tool as a local html file with Internet Explorer 11 on Windows 7 (SP1 32bit Jpn). If this tool won't work well on your PC, and you want to fix it, try it at your own risk. The script is below.
The script text is below.