Oh dear what a situation to be in. 
I agree with @Sandra.  A hem on this sort of dress wouldn't be wide.  Releasing side seams won’t alter the length unless it is so tight it will all fall a little.  And I wouldn’t want to mess with silk as it does show holes easily.   Perhaps releasing the empire seamline might give you a little length but not worth bothering with and the holes in the silk would scream at you!   Not sure how professional these people sound to me.  Tbh when making garments any fittings should be done with the client wearing the exact underwear and shoes that will be worn on the day.  It makes a huge difference.  It is not possible to know if a garment is going to fit properly otherwise.  If this is the toile it looks to be a bit tight to me anyway. 

Are you sure it is too short?   You don’t want it so long that you catch your shoes and  trip on the dress! 

Can you look at their terms and conditions?   I’m not sure I’d want to pay for something that in my opinion hasn’t been done properly from the start. 

Beautiful work @BrendaP and a great way of dealing with what is possibly the worst time most of us have lived through.

Live Really Useful Boxes.  Use them for my threads.  My current projects usually sit on my cutting table. 

Oh don’t that make you cross @Surest1tch ?   
We have a dishwasher but still the cups, glasses etc  often only make it to the worktop....on top of the dishwasher!!

Can you block fuse some very fine interfacing to it before cutting anything out?
I use a lot of this stuff.  It doesn’t alter the weight or hand of the fabric. 

https://englishcouture.co.uk/shop/fine-sheer-fusible-ecc21-fusible-interfacing/

The Singer Heavy Duty machine is rubbish.  We had one in the shop.  I got to try it and was asked to test it.     It certainly doesn’t do what they claim.  For this sort of work I’d go for a metal bodied machine.  TBH  modern Singer machines are all pretty rubbish.  There are much better machines around.

So glad it’s not just me @Ploshkin.  I pull my machine right forward.  But I’m short so that’s my excuse.     And before anyone says.....
Yes I can raise my chair higher.  But then I can’t reach the foot pedal.     I probably need a platform under the bench.