Hi Natasha :)
Long post ahead. Other readers can view a TL;DR down the bottom.
I asked a question of you some months ago regarding red patches on the ears of Dinah the mouse. Unfortunately, the patches turned out to be a secondary problem, as they disappeared quite quickly and attention turned to a more worrisome issue.
For approximately 1 month, Dinah began panting softly and consistently. She ate, ran on the wheel and played just as readily as her nestmates, but I noticed her eyes were bulging and she was steadily losing weight.
By the end of the month, her fur was well-groomed but craggy and the poor girl was skin and bone. I regret to say that I didn't have to funds to take her to the vet, and could only wish her a calm and swift death, which soon occurred.
During the month when Dinah was ill, I eliminated several possibilities for the cause of her illness and researched heavily. I discounted allergies (I've owned a mouse allergic to bedding and certain foods before, and her symptoms did not match the previous mouse's) and I called the vet to ask about your suggestion of sarcoptic mites, just in case the issue might be related, which I was told it was not. I now believe that it had something to do with the frequency with which I changed the tank's bedding. The mice have a very well-ventilated tank that is more than big enough to house the seven (once eight) of them comfortably, but I was struggling to buy the larger packs of litter that could cover the whole tank, so I compensated with shredded newspaper. I tried to change it once a week rather than twice a week, but while the less sensitive mice coped well, Dinah did not and the excess ammonia may have given her a chest infection.
Coming to this conclusion broke my heart, knowing that even though I was struggling financially, I had neglected my mice in favour of paying for other expenses. Luckily things have improved for me and therefore for my mice, and I've resolved to budget to accommodate my mice above less important things regardless of my financial situation, however in the meantime, there is one more mouse who now needs my attention.
The rest of the mice are strong and healthy despite my brief neglect, save one who is showing the early signs of a chest infection, herself. This should not happen with the others as I have rectified the bedding issue, but I would like some advice as to how to nurse Blossom, my big beautiful doe, through the next week or so before I can take her to the vet for some proper care. She is panting and her fur is craggier, but she eats well and is just as active as ever. I've heard that encouraging a sick mouse to eat echinacea drops with bread and lactose free milk can ease their symptoms a bit. Is this true? Do you have any advice for some home-care in the meantime? Blossom is a much bigger, stronger mouse than Dinah ever was and less sensitive too, so I believe she won't suffer decline as quickly.
Despite my guilt at Dinah's passing, I believe she managed to live as long as she did despite being sick by good food, company and activities, so I take heart knowing that I did all I could to make sure she died peacefully.
TL;DR: Sensitive doe passed away from a chest infection due to my ignorance, second (stronger) mouse showing signs of illness, too. Need advice to care for ill mouse before the vet can see her.
First, you're beating yourself up over something that you really have no way of telling is your fault. Newspaper, when changed regularly, is an acceptable bedding - Yesterday's News is a popular pellet brand of bedding which is both safe and composed of recycled newspaper, for just one example. The only difference is that newspaper is less absorbent, so you need to change it as soon as you can smell something. Not doing so causes ammonia build up, which compromises air quality for ALL mice, but does NOT cause a respiratory infection, which is a fairly common cause for panting and general decline in health. URIs are also very contagious, which may even explain why Blossom is panting now. Also, without a necropsy and histopathology, there is no way for you to be sure nothing else was going on - genetics play a big role in a mouse's lifeline, from internal tumors to weak cardiovascular or immune systems to organ failure. The only person who can tell you why your mouse passed on is a vet. Right now, I would spare yourself the blame, because while we all strive to be better pet owners, you are doing what you can and you clearly care a great deal.
On to your actual questions: I am not knowledgeable on homeopathic remedies for mice and cannot help you with the suggestions of echinacea. Stale bread is a good way to encourage a lethargic mouse to eat, but beyond that, is not directly helpful in any sort of infection or physical stress. Lactose free milk (if you try this, I recommend kitten milk replacement) is a quality protein source, but can cause loose stools and thus dehydration in adult mice, so I would restrict it unless pairing it with some other high-protein food such as mixed in with scrambled eggs or dusted over kitten kibble to bulk up a diet. Keeping her diet healthy, strong, and always available, along with ample water and a stable environment (avoiding temperature extremes, keeping the bedding sniffable, and providing exercise) is your best supportive care for the next week or two.
The most important thing is to identify what is causing the problem - if she has an upper respiratory infection, timely antibiotics are her best bet, and the best protection for the rest of the mice in the cage who have been exposed. You can even start her on tetracycline, an over the counter general antibiotic which is usually available in pet shops for fish, to hold her over until a vet can give a more accurate diagnosis and a targeted medication. I can help you with dosing instructions for the fish tetracycline if you'd like, just let me know. You may even be able to call your vet for their professional opinion, which would be better than mine, on care between now and an available appointment.
I hope I answered your questions fully, but if I missed something or you need any further help please feel free to write me a follow up! Best of luck to Blossom, and hopefully she feels better soon!