New T5i vs old 400D , pics no better on T5i ?

I am just learning about the difference in the resolutions on these cameras. I differed on my calculations for the difference from previous answers to my problems . I get it at
1.34 x 3888 = 5210
1.34 x 2592 = 3473

5210 x 3473 = 18094330 or 18mp ? I am guessing this is correct ?

I also need to learn about the STM lenses . I bought a body only camera as I thought my lenses would be fine for video and the T5i. I did not know that the silent focusing would come into play along with any other differences .
I did take a few short videos with the EF-S 17-55 1:2.8 IS USM , indoors .
The lens's focusing did seem to jump when following my dog run away from me . It hesitated in a blur while focusing when the dog moved about 20 feet away . and I now notice the sound of the lens focusing motor .
My wife who has been playing with video cameras for years compared it to her HD Canon Vixia HF 100 . She was impressed with the quality of the video , other than the sudden refocusing / blurry change as the dog ran away about 20 feet or so it was very clear and sharp ! Almost like real HD TV.

I have not seen a STM lens or it's results as of yet .
Does the same refocusing thing happen when tracking a subject ?
Do you hear the lens motor ? I will be using the video once in a while but not nearly as much as stills .
It is already a pain to have to change back and forth from my 2 favorite lenses( I am taking pics as they happen ) without having to go to 3 lenses at family events etc. ( When playing around with landscapes it is fun ! )
Indoors, in close spaces such as 14 x 20' rooms , I would like to have just one lens for all lol . I guess I am asking for too much but really want to know if there is a great difference in the STM and older pretty good USM lenses for video . IE glass vs focus motor .