Are the five buttons supposed to be mechanically the same?

Do the buttons work? 

On my rev C the bottom left button felt really mushy compared to the others. Especially the top left and top right buttons were really clicky in comparison. I have a rev D now and the down button feels totally different. 

There are people saying that "if it works it's fine". I don't agree, first the watch was €700 so they'd better work properly, but more so that when biking with gloves on the mushy buttons don't give any noticeable feedback of being pressed, so I had no idea the click was actually registered correctly. With a clicky button there is no such issue.

The rev C and rev D bit is a red herring. it's the revision of the box, not the watch.

Well I read Garmin Blake stating that hardware changes had been made to the 945 to have better buttons.

I have a new 945 now due to my old one dying on me and it definitely has more clicky buttons.

Boxes are identical though between the c and the d so I wouldn't know why it'd be a box revision indication.

Check the bottom half of your box. Is it Rev D as well?

I still have both boxes and unless it is merely fixing a type somewhere on the box I cannot see any difference between the two, apart from one stating "rev C" and the other "rev D" on the bottom corner.

It probably is merely fixing a small error, but that doesn't matter. The rev status printed on the box is associated with the Garmin part number it is printed next to, starting with 007. That part number is different for the top and the bottom half of the box, and it is likely the revision status of the two parts are different. Your watch itself has a P/N starting with 010.

I'm not saying Garmin have not changed the hardware in the watch. I had a very early one and changed it after about 10 months because I was not happy with the buttons and my new 945 is much better.  However it's the serial number of your watch which determines what hardware version it is, not the revision of the box.

I've never had a Garmin watch where all the buttons are the same resistance.  As long has my "lap" button clicks because I can't physically look to see if it registered while running speed intervals, I don't give a rip about the others as long as they work.  That said, Garmin's quality control, or lack thereof, has always kind of baffled me.

I'm on my second 945 and was firstly very happy with the buttons but after four months they are the same trash as of my first 945. Seems a quality issue with the buttons.

The serial number of the watch is the one printed both on the lid of the box and on the back of the watch, right?

How can i determine from the serial number (of the watch) what hardware version I have? And what is the version (if any!!) starting from which there tend to be no button issues?

In the top (outer) half of the box it is reported (as I initially said) revision C. But in the bottom (inner, ie the side that touches the table if the box is standing on a table) half of the box it actually says revision A.