You must be so frustrated!!!
I hope that all will get back on track on Friday and that the chemo will make a difference.
If you find that you do have to put her to sleep, I can tell you how it has been for my cats. I've had to put down quite a few over the last 15-20 years, as I'm sure many of the PT'rs here have. Most of my cats were very old at the time. I've been fortunate to have the vet come to my home on some occasions and it sounds like this would be available to you as well? This is much more peaceful I think for both the owner and the pet.
I have always asked that a sedative be given (by injection) prior to them being put down. Within 5-10 minutes they become very drowsy and during this time I cuddle with them. When they are almost asleep, the doctor shaves their arm and gives them the final injection. It takes only seconds.
It is still very hard, very emotional and it takes awhile to get over it.
Please know that if you decide to do this, we're here for you.

On a brighter note... I pray that the chemo does it's job and that Sydney has an amazing future with you! Don't give up hope.