It should work with it I use mine with a Canon 5DMK3 24-70 zoom lens and it is more than capable of handling that. If the Nikon D750 is of a similar size it shouldn’t be a problem at all. In a nutshell it handles large DSLR is very well it’s very robust and the build quality is excellent definitely worth a try. 

No. You have to purchase those separately. I purchased another tripod so that I could had access to both handles at all times. Swapping the tripod from the bottom to the top is a pain so much easier to have both attached, and the added weight is negligible. BTW this is the best gimbal I have ever owned. The Ronin S is heavy and way overrated, Zhiyun Weebill S all day. 

yes, the Weebill S can support your camera, these fuction is in the Weebill S, so you can use it once you can balance your camera on the weebill s, but you also need to know that you can't control the the d3500 via weebill s.