How to "merge" syncs? This is my problem: I had a tablet, and it was never synced to its Google account. I broke it, and I bought a smartphone, on which I activated the sync on the same account. In the meanwhile I got the tablet repaired, and now I would retrieve all the old data.

The question is: if I sync the tablet, the data on the tablet (whatsapp conversations, calendar events, contacts etc) will be deleted or merged? And if they will be deleted, how can I merge the old data to my new smartphone?
At least for contacts it's easy: the contacts app offers "export to SD card" (resulting in a .vcf file) and "import from SD card" as well. I'd do that first anyhow, so if something goes wrong you still have that.

The calendar app unfortunately doesn't offer a corresponding option – but there are some calendar backup apps which should be helpful (check that page for other backup apps as well to cover things you might have forgotten to mention).

WhatsApp will be trouble here I'm afraid.

Apart from the above, calendar and contacts should merge fine if you use either Google sync or your own DAV resource. Still, it cannot hurt to have a separate backup :)