When starting a game offline and the sync data loads, could it retrieve my data to keep going?

to sync the data you have to be online, the thing is that you can play it offlline but be sure eveytime you star the game you're online, because if something happens to your xbox, all data that is not sync with the could will be lost

Every game keeps two copies of the save file: one locally on your hard drive and one in the cloud. When you start a game when you're offline, the console will just use your local save file. When you're online and start up a game, your console will check
to see if your save files locally and in the cloud match; if they can be reconciled, the game loads as normal. If there's a discrepancy, the console will ask you to select which save file to keep: if your main save is the one you play offline, you select to
keep the local one and then it uploads that to the cloud so the two saves are in sync again; if you select the cloud-based one as the one to keep, your console
downloads it and overwrites the local one so the two saves are in sync.

By playing without an internet connection you're just using the local save file. When you're online, the console is uploading the latest version of it to the cloud so they're in sync. One isn't better than the other and as long as you're always on the same
console, there will never be a problem. It doesn't matter if you play offline most of the time or if you're always online.

I hope that answers the question.

You have to be online to sync data with the cloud. Your local save is always saved to your console. If you don't sync to the internet when you haven't in a while, if you somehow lose your saved data you can not sync it back, but you rather get the last synced
data sent to the cloud.

For more info see here:
https://support.xbox.com/games/game-setup/cloud-game-saves-faq

Just resumed play after work but am getting a message saying "Syncing Data for Halo Master Chief Collection" followed by "Unable to retrieve latest save game data. Try again or continue offline."

If I continue offline, all my cross-game achievement progress will be lost won't it?

In this case try to:

• Delete the local data
  
• Disconnect your console from the Internet (essentially go offline)
  
• Load the game and start a new game.
  
• Save your game when you get the chance
  
• Close down the game
  
• Go back online
  
• Start your game game again. When prompted to sync your cloud save, cancel the sync.
  
• Continue to to play the game and save.
  

Creating a new game offline, saving and then stopping the sync when online will create new save in the cloud overwriting your old one.