I understand. I am of the "better a day early than a day late" crowd, but that is stinking hard to stick to when it's your own pet.
For me though, I've only put down the actively ill. The cancer riddled ferret, the cat with renal failure, or the kitten with FIP. When I let those go too long, the end was horrific and I wish I'd taken them in a day before rather than have them feel that terror and pain
. I've never had to help an elderly pet pass on.
I do think we owe them dignity and as much time as we can. For now, I would stay put her in the bathroom when you leave (they have super nice beds for arthritic, older pets now... I'd spring for a cushy one). Keep a journal of how she does each day so you'll be more likely to notice a pettern of change or decline. If it won't distress her, I would take her food and water up after a certain time each night.
You might also consider giving her a puppy pad in the bathroom. If she's always been housetrained, she's probably so upset when she can't help it. I've had a dog left too long between bathroom breaks go in the house and oh, the shame on her little beagle face. They don't want to go inside either. Course, if it's that she's unaware she needs to go that's different, but it sounds more like she just cannot hold it like a younger dog can.