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[1] It might if dropbox was a more lean operation, but it isn't.
I may be misunderstanding your question, but it seems to me that this is literally the whole point of the service. Dropbox is a folder that syncs. You change something on one device (one write), it updates everywhere (multiple reads).
They do have some tricks to lower their bandwidth usage. AFAIR, if multiple devices are in the same LAN, only one gets synced with the cloud, and the rest sync to that one. Also, they have (recently introduced) a sync option, in which fake files are visible in your file system, but no actual contents get put on the drive until you try to open them.
I thought that was a first in the earliest of versions and was one of my favorite. At some point they removed it and made you choose which folders to sync. At which point I abandoned using it. So maybe they are reintroducing it or I am misremembering something.
(I think a good Linux analogy would be FUSE, things like sshfs.)
The postal system solved this problem ages ago, you charge the sender postage. Done.
Nope, just confirmed again. The only way around it is to go with a business account, which even the cheapest option costs 2x the price of just you and your wife both having Plus accounts of your own.
It's a solution not without compromise in terms of management and accounting but does achieve an outcome that would fit the apparent need.