I didn't realize that Nikon handled setting ISO so differently on different models. My two bodies have a button dedicated solely to setting ISO. The manual you should have received with the camera says to push the "i" button (is that what you are referring to as the "function button" and a menu will appear with all the setting that can be adjusted. The "multi selector" is then used to choose the setting you wish to change, and finally the "OK" button will show you what options you have to choose from.
Once you choose the desired option (in this case the ISO from the menu offered) it should not change once you set it - - - unless you have somehow landed on "auto ISO" which allows the camera to decide what the setting should be. I virtually never use Auto ISO unless the light is constantly changing and I can't be bothered with changing the other settings

I hope this helps.

Set your camera to aperture priority and trust your cameras light meter. Of course you could use auto mode and hope for the best. I would shoot at 35mm since yours is a cropped sensor camera. Sounds like the sun will be an issue so use the lens hood. If the pic is a little underexposed or overexposed you can fix it in post. Take a full length shot of the subjects with the trucks in the background. Then move in for a closer shot maybe from the waist up again making sure the trucks show up in the background. Now, take a third shot with just the two people handing the keys off. Make sure your composition is good. No cut off heads, hands, etc. What ever you do make SURE you are steady and in focus on the CEOs face! Flash if faces in the shade. Never shoot with the sun in your subjects face as all you will see are squinting eyes, NG! Let your boss know before hand you would like to take three shots so he can choose between the three and you will be a hit! Take a deep breath and go for it! It would be great if you would post the pics and comment on them. Good shooting to you!

You will probably want the CEO and at least one truck in the picture. Being right out in the sun has advantages and disadvantages. You would have plenty of light, but the light might be harsh. Especially if the light is coming at an angle. If it is bright and the truck and people could be in the shade of the warehouse (you know with the warehouse sign in there too), you might have enough ambient light to take a nice shot. If you are not in the shade, keep the sun behind you if possible.

After they hand off the keys, ask them to get a little closer, no closer than that, NO! A LITTLE CLOSER and then say "and now you may kiss the bride" Then, hopefully, get a shot of them laughing ...or chasing you either works.

First something quick and dirty: In your post process software, brighten the top photo up a couple stops, then add some vignette to darken the corners.

Now, in your camera, add some diffuse flash, and under expose the ambient image (if your camera calls for f/8@1/200, go into manual and set your camera to f/16 @1/200). This will darken the background and brighten the main flower

For the smoke with the sparkler, set up a box fan at your feet blowing towards your daughter to blow the smoke behind her.