Thanks so much for the answers to my questions, but I'm afraid my response to you isn't going to be very encouraging and I'm very sorry for that.
When dogs present with pale gums and weakness, then I worry about anemia. If you'll bear with me, I'll discuss the most common causes of anemia in a dog this age:
1. Increased destruction as would be seen with an auto immune disease (Autoimmune Hemolytic Anemia) which means the body is destroying red blood cells because they are recognized as "foreign".
For a dog this age who might develop this condition, I am cautiously optimistic that they can successfully be treated but high doses of steroids are the required treatment before improvement might be seen.
2. Increased sequestration which means that the red blood cells might be collecting in the spleen. This is often secondary to a cancerous process, I am sad to say. An ultrasound would be useful in determining if the spleen is enlarged and an aspirate or cells or biopsy could confirm if this is cancer.
Sometimes the spleen can rupture and cause an abdominal bleed. If the bleed is slight enough, then the body can resorb the blood until the next event. But if the bleed is significant, it may take longer for the blood to be resorbed...days instead of hours or minutes. And t sudden death can also occur.
3. Decreased production such as a primary problem with the bone marrow. This is often associated with cancer, as well, I am sad to say. It would take a bone marrow biopsy to confirm.
4. Many tick diseases can cause anemia.
We do have tests for many of these diseases, but we are testing for antibodies, not the organism itself, in most cases. If antibodies are not being produced (for whatever reason) then the tests could be negative and yet this is still the underlying problem.
I've also come to believe that there are tick diseases that we haven't even been able to identify yet and thus would not have tests for all of them.
Doxycycline is the drug of choice for most tick diseases that we see.
5. Depending on the level of anemia, chronic disease can also cause levels to be low. This is often seen with kidney disease but any chronic disease process can cause it.
Other possible explanations for weakness, pale gums and weak pulses would be cardiac disease. Without an ultrasound, then a definitive diagnosis can't be made although an x-ray may be suggestive of this condition. This could be either a primary problem or secondary.....unfortuantely, cancer of the spleen (usually hemangiosarcoma) can also spread to the heart and cause issues there as well as in the abdomen.
Given that she had a seizure two days ago (and I'm assuming that this was a first time event), in combination with her other symptoms, then the likelihood that Mika has a very bad condition is pretty high, it saddens me to say.
As far as diagnostic testing is concerned, bloodwork, x-rays and an ultrasound are not invasive procedures. She wouldn't have to be sedated for them and they shouldn't weak her further. Obviously something like a biopsy would be more invasive.
The only antibiotic that I might consider in a situation like this would be Doxycycline on the off chance that this is a tick disease. Vitamin B might help her feel a little better but is not addressing the underlying problem.
Without additional diagnostic testing, then the cause of her current symptoms is impossible to know for sure. But I would be doing a disservice to both you and Mika if I were overly optimistic about her situation at this point. I know this is not what you want to hear and I'm terribly sorry about that, but I hope you understand.
I also hope this helps you to understand the various possible causes of her signs.