As i have to make a lot of AD/DA conversion I supose that sample rate/freq is very important to conserve a maximum of analog stuff benefit?

The easiest way is to check that for yourself is with the 8 outs you have and run the same mix at 44/48 and 96.
Yes summing can add something, but it can also destroy something and at best it is subtle. Sometimes placebo....or just a slightly different mix. Only real reason to do it is if you like the hands on fader, or if customers demand it . A very good summing box (a Neve if I remember correctly) was tested by sound on sound. Yes it did add some magic, but also when just used as outboard on the master out. It was not the summing but the transformers.
What are you using for summing?

And 24/44? No reason to work at 16 bits. (if it´s not  a typo) and maybe even try 48Khz, might just make a small difference.
BTW I have an ADI8 AE which has the same output converters as the DS (AFAIK). I like them...

Yes you can work with 8 channel 24/44 and 24/48.
Typo (typing error), I thought maybe you meant 16 channels instead of 16 bits. No reason to use 16 bits ever, except when delivering the final finished dithered master for cd or mp3.
Hope this is helpful.

Yes maybe, but so does going from 96 to 44 and even 88 to 44. It´s just what you like best. Sample rate converting is also work, so it must have some benefit to do it. But Since you are going outboard 48K might be useful especialy if you can record that mix on another good interface at 24/44. Just a small test will tell you.
Dithering from 24/44 to 16/44 will create no issues at all (but needs to be done right). Only sample rate conversion does, but if it´s audible using a good program??