Are these alterations possible?

If they’re going to remove layers, they would likely take the entire skirt apart to remove it, not just slice it off at the seam. The thicker layer is likely giving the dress the A-line shape so you may want to consider that in deciding to take it out. It could also cause your skirt to be more transparent.

If all you are wanting is one layer of tulle removed from the under skirt, that should be super easy! As a matter of fact, I removed my own! There was one stiff, starchy layer of tulle that was making my dress just a little too pouffy, so I just cut it off as close to the seam as I could get, and it worked out perfectly! My mom thought I was nuts, but I just couldn’t see paying a seamstress to do it (or worse, charge me to take the entire skirt apart!) when I could just cut it off myself for free in two minutes 🤷🏼‍♀️

I definitely see the changed you want being possible.

If you remove the stiff layer but find it loosing shape or you need some more leg room I would suggest finding an A line hoop skirt, it would be removable after the ceremony but would allow for extra leg room.

Is the stiff layer you mentioned made out of crinoline by any chance? My dress has a crinoline under layer to help create the a-line shape but the boutique is going to make it detachable for me so it seemingly is possible! Seamstresses can work some serious magic haha.

If you are short, and the dress needs hemming, it appears from these pics that at least one of your layers has a curved or scalloped lower edge. That can not just be turned up to hem it. And with 4 layers, you would be hemming like 800 inches. So the way it is usually done, is to unstitch the lower zipper, remove the 4 layer skirts. At this point , the inches you need removed are taken a little from the upper body, so waist seam is at waist. And the rest by cutting a few inches off all the skirts. This is the time to remove the crinoline/ stiff netting layer. If it needs a lighter softer one, or a lining, the seamstress can put it in. If not, the skirts will be tapered at the new waist and gathered, without the crinoline. It is wise to have the seamstress adjust the length, and put elastic or a drawstring on top op the crinoline, Now it is a removable petticoat. You might want to put it on, as you would a half slip, for a couple of formal portraits.
Or if it is hot and humid, and you want to slip on a net that will push droopy lace away from your body.
But removal during hemming is easy.