Yup, all the 'more private' ones like SimpleX or Session don't have as many features and are have pretty suspicious funding sources (or involved in crypto).
WhatsApp might be private but there's no way to be 100% sure, since it's closed source and can leak some messages in reports which is intentional.
Yes and no about Signal supporting multiple Android devices, the Molly fork might be able to if you set it up as a linked device, and we know that apple tablets can be linked in the actual Signal app.
3, 4, 5 and 11.
Not sure about 1 as we run it on our tablet so it definetly can be linked to other devices but we aren't sure how many.
7 can be done by linking devices, so not sure what is meant by this.
Edit: ah, okay, so seems the Signal app itself cannot be linked to multiple Android devices, including tablets, that's pretty bad. The Molly fork can work as a linked devices though, however Signal only supports linking 5 devices at the same time.