Did AES Crypt not work? If you need to do it the harder way, here’s an example of decrypting a .zip.aes file into just a .zip file (which you should then be able to open with any zip program, but insides aren’t really for humans to be interpreting. Use a Duplicati installation or one of the standalone restore scripts for that part.
C:\Duplicati Backups\local test 7 encrypted>dir Duplicati-b*
Volume in drive C is Windows
Volume Serial Number is 5822-1128
Directory of C:\Duplicati Backups\local test 7 encrypted
12/25/2018 02:17 PM 1,005 duplicati-b4574aeb7200248b8a4ed0473b6836192.dblock.zip.aes
11/27/2018 11:57 AM 1,277 duplicati-bbaa0dda752734c749b07a894c0fd5c43.dblock.zip.aes
11/23/2018 03:36 PM 1,437 duplicati-bea036779eba84c48921bd4a3d65f122e.dblock.zip.aes
3 File(s) 3,719 bytes
0 Dir(s) 35,302,105,088 bytes free
C:\Duplicati Backups\local test 7 encrypted>"C:\Program Files\Duplicati 2\SharpAesCrypt.exe" help
Usage: SharpAESCrypt e|d[o][1-4] <password> [<fromPath> [<toPath>]]
Use 'e' or 'd' to specify operation: encrypt or decrypt.
Append an 'o' to the operation for optimistic mode. This will skip some tests and leaves partial/invalid files on disk.
Append a single number (up to 4) to the operation to set the number of threads used for crypting. Default is single thread mode (1).
If you ommit the fromPath or toPath, stdin/stdout are used insted, e.g.:
SharpAESCrypt e 1234 < file.jpg > file.jpg.aes
Abnormal exit will return an errorlevel above 0 (zero):
4 - Password invalid
3 - HMAC Mismatch / altered data (also invalid password for version 0 files)
2 - Missing input stream / input file not found
1 - Any other cryptographic or IO exception
C:\Duplicati Backups\local test 7 encrypted>"C:\Program Files\Duplicati 2\SharpAesCrypt.exe" d password duplicati-b4574aeb7200248b8a4ed0473b6836192.dblock.zip.aes duplicati-b4574aeb7200248b8a4ed0473b6836192.dblock.zip
C:\Duplicati Backups\local test 7 encrypted>dir Duplicati-b*
Volume in drive C is Windows
Volume Serial Number is 5822-1128
Directory of C:\Duplicati Backups\local test 7 encrypted
04/29/2019 04:32 PM 689 duplicati-b4574aeb7200248b8a4ed0473b6836192.dblock.zip
12/25/2018 02:17 PM 1,005 duplicati-b4574aeb7200248b8a4ed0473b6836192.dblock.zip.aes
11/27/2018 11:57 AM 1,277 duplicati-bbaa0dda752734c749b07a894c0fd5c43.dblock.zip.aes
11/23/2018 03:36 PM 1,437 duplicati-bea036779eba84c48921bd4a3d65f122e.dblock.zip.aes
4 File(s) 4,408 bytes
0 Dir(s) 35,300,982,784 bytes free
C:\Duplicati Backups\local test 7 encrypted>
That’s enough to give some confidence that a typical dblock file decrypts OK. Using AES Crypt would be easier than doing this for each file, but at least one little test adds confidence that all the files will decrypt.
If you’re on Linux or Mac (are you?), you might need to run mono. Do you have only one file downloaded?
EDIT: I’m trying to learn whether or not it’s even possible to download everything before your deadline. To sample can give confidence (and more sampling can be done), but you may be download-speed-limited. Using a faster connection and downloader may help, but it might be a race against time (or a gamble that everything will wind up OK after whatever preliminary testing can be done before some deadline passes).
EDIT 2: Do you have an idea of how much OneDrive space your backup takes, and your Internet speed? Multiplying size in bytes times 8 to get bits, then dividing by your connection bits-per-second (probably in millions, so factor that in) will give you seconds theoretical-best-case-possible using parallel downloader.