Can I 100% transfer data from a dead Mac to a new one?

Did you create a backup of your MacBook Air, using TimeMachine? This would be the only way to get your data back, if your MacBook Air does not start up at all.

Getting the MacBook Air checked by an Authorized Apple Service Provider might help, in case the problem is related to the battery.

Apple - Find Locations

Yes, without a backup your data can't be accessed. But, depending on your model, an Authorized Apple Service Provider might be able to extract the SSD and use it as an external drive.

So, contact your local Apple Service Provider, they can tell you exactly if this would be an option.