Remote and Local Databases advices

I have a project on my mind and I wanted to gather some advices to realize it.

The project is a managing application designed for computers, iOS and Android, and I have some troubles finding the best way to store the datas that are managed.

I first thought that LocalDatas would work, but if we own multiple devices (Computer and an Android for example) it will be a total chaos for the user to manage properly his datas.

So I decided to store the datas on a remote server. But now I have some issues finding the best way to do it, considering the fact that mobile devices (iOS and Android) may can't be connected to the Internet, through WiFi and even 3/4G.

I could store datas localy (SQLite) and remotly (MySQL), and then synchronize the "two databases" but I don't know if it's a good way, and how to perform this. Furthermore, I can't see how I can deal with "multiple synchronisations" :

Imagine we have somebody who owns a tablet and a mobile. This person give his tablet to a friend or a colleague, and they (both at the same time) make changes to their localdatabase, and once they're done, they push the local database to the remote one at the same time. I am pretty sure this case will create many conflicts on the database...

Finally, what about JSon ? Is it reliable to store a quite big amout of data ?

Thank you for your replies,

PS : Sorry, English isn't my native language so there migth be some mistakes in my message but I hope you'll understand it anyway.