Q: Can I pull data from Google Sheets at runtime?
A: No, Unity-Quicksheet is a tool for easy handling of data at edit time. Saying with the technique point of view, all data retrieved from Excel or Google spreadsheet is stored form of an asset file as ScriptableObject derived class object to make it easy for serializing and deserializing.
Unity's ScriptableObject is not a one for persistence data which means there is no way to serialize any runtime changed data. Unity-Quicksheet is not invented for that purpose. So if the data you have can be changed at runtime and should be persistence, you should consider other thing such as json which is being for that purpose.
There are bunch of formats for bidirectional serialization e.g. JSON, BSON or XML etc. The decision entirely depends on your platform and/or requirements.
On the other hand, there is no reason to get data from Excel at run time on most of cases. Not likely Excel, it may useful to connect and get data at runtime from Google Drive but to keep the same interfaces with Excel, Unity-Quicksheet does not provide that for Google spreadsheet either. (Though it may be changed in the future)
Q: Does it support all platforms?
A:Yes, it can run on any platform specified as Unity's build target platform except one restriction, Google spreadsheet on Web Player build target setting.
Google Spreadsheet plugin does not work in the Unity web player’s security sandbox due to the security restriction on Unity side. So you should change the platform setting to ‘Stand Alone’ or something else such as ‘iOS’ or ‘Android’ platform in the 'Build Setting' to make it work.
Q: Can I save any changed data from Unity editor back to Excel or Google spreadsheet?
A: No, you can't. Keep changes on Excel or Google drive and make one way for the data flow is better for error proof.
Q: Can I get data from Excel file on OSX of Mac machine?
A: Yes, it runs on Mac machine without any problems. Keep it mind that save your excel file as '.xls' not as '.xlsx' file on Mac machine otherwise you may get the error, ‘Can’t read content types part!’
Q: Can I use 'Open Office' instead of Excel?
A: Yes, both of them get same result for '.xls' file.