Thank you Alexandra,
Now I am truly sorry to hear that your lass is being bombarded by multiple issues here. While I do think it was fair to try and clear her GI signs and stabilize her heart to see if she could spend more time with you; her current state does make me concerned that she is not responding to treatment and that we do need to consider letting her go at this stage.
The reason why I do think that we may be at that is point now is because her respiratory rate is severely elevated and she sounds to be decompensating despite being on treatment. (Her breathing rate should actually be ~ 20-30 breaths per minute, so 100bpm is a serious severe worry). Furthermore, to see this elevated breathing rate coupled with pale gums tells us that she isn't getting enough oxygen despite breathing so hard. And while her lack of appetite is a serious issue, it may be that she is so focused on breathing that she just cannot spare the energy, focus, or breath to eat. And this low oxygen will make her weak, wobbly, and likely the cause for her being slow and laying down while drinking. So, to hear that she is in this state and struggling with taking in oxygen and getting it into her circulation despite being on Vetmedin really raises worries that her heart disease is already too far for us to manage.
So, while I would say that we could consider addressing her nausea and GI signs with an antacid [ie Pepcid (More Info/Dose) or Zantac (More Info/Dose)], light diet (ie rice with boiled chicken, white fish, scrambled egg, cottage cheese, etc), +/- Kaolin/Kaopectate (More Info/Dose) or Protexin Pro-Fiber (which is available OTC at vet practices; example) for the diarrhea; I am concerned that those GI signs are a lesser issue for her. Because even if we can settle her GI signs, we still have a struggling heart and lungs that are not getting oxygen into and around the body as they should.
Overall, I am very sorry to say that I am very concerned about your lass's current state. Her GI issues sound to be a lesser concern here. Instead, she sounds to be in a deteriorating condition where she is breathing so fast but yet cannot get enough oxygen to keep herself doing those things important to her life. Therefore, it sounds like you have tried but her situation is already too advanced. And as she is struggling despite treatment for her heart, I do think this is a situation where we'd need to consider letting her go to keep her from suffering.
Finally, since she does sound in a severe respiratory state, I do have to note that this would be something to consider having seen urgently. So, I do want to note that some veterinary practices in our country have office hours over the weekend. As well, I wanted to mention that most veterinary practices here do have contingency plans for emergency care for their patients even when they are not open. Therefore, it is worth ringing the practice as they will likely have a message to direct you on how to contact their out of hours service. And if you don't have an open vet you can find one local to you, you can check the RCVS Register (HERE) or Vets Now (LINK) who are open all nights/weekends. In any case, if she is really struggling we'd want to use one of these options to have her seen now.
Please take care,