Is there any way to move a whole project back to 2.0.8?
I do this regularly with all my forum code samples and it works. The main issue, much more severe than the Combobox.ReadOnly thing, is the change in the file structure of the project's xml files. When you plan to port a project back to non-trunk you MUST go to "Project Options" > "Miscellaneous" and check "Maximize compatibility of project files (lpi and lps)" - this writes files in the old structure. Otherwise the project will look as if it does not contain any forms.
If you write packages you must know that a similar switch exists in the options of a package: At the bottom of the first page of the Package Editor there is a "Maximize compatibility of package file (lpk)" which must be checked, too. Note that this requires a recent trunk version because there was a bug which was fixed only recently.
The compilation issues of the specialized comboboxes in TAChart and FPSpreadsheet have been fixed. I cannot comment on FastReport 6 - you should contact the vendor.
The ReadOnly property of TCombobox has been deprecated for a long time. Read
https://wiki.freepascal.org/Lazarus_2.0.0_release_notes#TCustomComboBox.ReadOnly_was_deprecated to learn about the reasons.