I love the shape of your dress as is and wouldn't change a thing!
If you want more volume you could try on different petticoats or talk about crinoline options with your seamstress, but it's totally up to you! I was on the other end of the spectrum and had wayyyy too much poof in my skirt so I kept cutting layers of crinoline out of my dress
My dress comes with a lot of crinoline under it, so no. I'm also having them put a little crinoline around like a bustle in the sides and back. Originally, I was thinking petticoat but if the dress can support it, having it put in the dress itself is so much easier.
I'm not using a petticoat as I am happy with the volume of the skirt. If you are happy with the skirt don't. Another option is you could wear a petticoat for the ceremony for volume and remove it for the reception!
I don't think it's necessary. I could have gone either way because I noticed a lot of brides who've worn my dress have gone without. I tried the dress at the bridal salon when it came in with a petticoat (or a facsimile since they didn't have one in stock) and I like how it poofs it out more. My dress is tea length so the petticoat gives it more of an old school bell shape which I like. Plus, it pushes the dress away from my legs so I'm hoping it doesn't get too hot!
I don't think it's necessary in all situations. I think it's more preference. I am wearing one but only because my dress is massive on the bottom and therefore it's pretty heavy but with the underskirt it lifts it a little and makes it a lot lighter and easier to walk around in.