Yes, if you are in desktop and choose to import to QuickBooks Online again, it will overwrite ALL the the existing data.
In answer to your 2nd question... to my knowledge, Intuit is not working on a way to sync desktop data to online version in realtime. You can always submit feedback and request this by going to the Gear Icon and selecting Feedback, but I honestly don't see this feature happening as they are 2 different programs designed for 2 different customer bases.
If you want the features of desktop but in a 100% cloud based solution, Intuit offers hosting services for QuickBooks Desktop or you can research your own solutions such as a dedicated computer for hosting and remote access software.
Additional information. You can only import desktop into an existing online company within its first 60 days of existence. After tgat you would have to start a new company subscription (And cancel the old if you only need one)
Only Enterprise allows combined financials from multiple companies but you can have all the companies you want in Desktop for free or as many in QBO that your wallet can afford.
Other methods rather than hosting to access desktop vompany on the road is a second install on a second computer for use only by the licensee, one computer are time and using backup/Restore to sync between or to use a remote desktop application