That's great about Arwen's healing. She sounds like a really healthy girl.
The idea of going back to the beginning is not a bad one. I don't think that you failed so much as you had a challenging student. But, even if you missed a step somewhere along the way, this is your chance to correct it, and with an older student.
I've been having to reiterate to the guys that, just because Tango was literally just out, and did everything, multiple x's, doesn't mean that he shouldn't be taken out if he asks to go back out.
And, 99% of the time, he does what he needs to...again.
However, they started giving him a fresh bowl water when he went out, or came back in, and he's a water hound. There's not enough water on the planet to keep him happy, and it's been that way all of his life.
Then we started getting into the issue of him wanting out for a freshly filled bowl of water, or a treat, etc.
So, I lined everyone up and said that we needed to take a breath, and undo the problem that was being created. And, that would be my advice for Arwen, as well.
The first thing, is to take her out on a leash...no playing. Make a separate playtime.
Second, keep the 'event' low key.
If she's having a lot of fun, playing, getting a treat, etc. she may do what Tango did for that while; use going out to get what he wanted, not only for doing his business.
Not that it's strictly bad to let you know that she wants something else, but mom gets tired of going in and out, as you said, and still getting a mess in the house. In the winter, it will be 100x's worse.
So, set aside a play time separately, don't give treats when you come back in, only outside, if you give them for that. Rather, when in the house, give them randomly, when she's being good/quiet, and have her do something for the treat.
She's young, and she's a smart breed, so she does need a lot of exercise, both for her body, and her mind. If you get her to sit, down, or whatever she knows already, then give her a treat, followed by quiet pets and praise, she will begin to change her ideas on what treats are for. She should find learning fun. And, it's good for a SD.
As for the indoor mess problem...
I would probably stay out...without playing, longer. She's obviously holding back to some degree, maybe on purpose, maybe not, to be able to make that many messes in such a short time after going back in.
I've had dogs do something similar, and they were quite active, so didn't want to put the time into doing it all completely. They we too distracted. When pushed, though, by being out there, and keeping a tight focus on sniffing and walking around only, they eventually did do more, though you wouldn't think that they could. They simply did not fully empty their bladders, etc.
Then, after spending longer out there with her only being allowed to sit and wait you out...which you won't let happen, or walk and sniff, until you get the success that you are looking for. Every time, she gets praise. Then, put her in her crate when you come in, without fanfare.
If the other animals' 'down time' could coincide with Arwen's, that would be ideal. After she's spent, say, 20-30 mins. resting in her crate, you have 2 choices...
A - Take her back out before letting her loose.
B - Take a chance, and just let her out of the crate, and see what happens.
If B gets you messes, then you need to go with A.
When you choose a play time, btw, frame the question you like best to her (ie: Do you want to play?), and take her out off leash, if possible, otherwise on a long lead, with a ball, or whatever she wants to play with out there. Don't worry so much about what else she does during playtime, but if you see her doing something, mark it with praise. If you can make a decent cluck with your tongue, that helps to mark the behavior, just before the praise, a lot like a clicker, but easier to use, and always with you!
Hopefully, if you start back with frequent trips out, etc., you will see a reduction of messes, and then the end of them altogether.
I don't like to crate after an indoor mess for 2 reasons.
1 - You don't want the crate to seem like a place for punishment.
2 - It's already happened, so won't serve a lot of purpose. She won't make the connection anyway, unless you caught her in the act.
In this day and age of phones that you can set alarms on, I would say to go back by setting an alarm for a certain amount of time, preventing human error of her having to wait to long to go out several times a day.
I also take pups out any time that they've awakened from a nap/sleep, after meals or drinks of water, after or even sometimes during indoor play, and before bed, of course.
Hopefully, all of this made sense. I've not had much sleep yet, so may have misspoken somewhere along the way. Hopefully not.
Love All Life, Thank You For Posting! :)