To 2: Yes, if the device with which you want to connect the repeater uses both frequency bands. This has worked for me so far with various products, including an Archer WLAN router.

To 3.: Mesh functions only work with AVM devices. Several 1750Es cannot form a mesh with each other if there is no mesh master from AVM.

What does work, however, is connecting a 1750E specifically to another 1750E instead of the WLAN router. This can be done in the configuration tool per band under "Access to Home Network" - "Individual Settings for the Wireless Connection to the Home Network". In my case, two 1750Es (B, C) should connect to another 1750E (A), which in turn establishes the connection to the WLAN router. In this case it even worked by WPS connection between (A) and the WLAN router with subsequent WPS connection between (B) and (A) and then (C) and (A).

I have not tried whether this can be cascaded under certain conditions (WLAN router <--> (A) <--> (B) <--> (C)), my (B) and (C) only ever see (A) and the WLAN router.

1. if so, only the connection from the repeater is slowed down/the maximum of the repeater is used.
2. I don't know, but you can connect to the repeater via IP and make various settings there. As far as I know, the network will be forwarded with the same SSID. If you have activated 2.4 and 5G on the router, both SSIDs will be listed again - so you can select your "line" for devices with WLAN selection.
I also operate it with a Netgear for 1-2 years, today with the Swisscom router .... it has not touched the repeater for months - when rebooting the router, plugging and unplugging the repeater ... is always directly connected.

I am very satisfied, but I don't know if the bandwidth, after a few years, is still comparable with newer devices - that would be my only point of comparison with other products.