I agree that this is a difficult case as she has several different problems which may or may not be related. It's always difficult in these situations when weighing quality of life and trying to determine when euthanasia is the kindest option. Unfortunately, they rarely ever just slip away and we are forced to make choices we'd rather not have to address.
Let's start by breaking down her signs:
Diarrhea-
The recent change to blood in the stool indicates that the problem, at least part of it, involves the large bowel. Generally in cases of colitis (large bowel inflammation) they experience straining, increased frequency of bowel movements, mucus in the stool, and sometimes urgency associated with defecation. There are many potential causes for this, anything from inflammatory bowel disease, infections (bacterial, parasitic) metabolic causes (particularly Addison's disease involving low blood cortisol and electrolyte abnormalities, and you mentioned a mildly enlarged liver which could contribute), or cancer. As a geriatric dog, your girl could be susceptible to any one of these things. Older dogs also tend to have difficulty absorbing certain nutrients, particularly fats and proteins. You didn't mention a significant weight loss, so I would think protein loss is unlikely. Your vet can submit a stool sample to analyze for excessive protein in the feces. Also, both R/D and I/D are low in fat, high in fiber, so that would suggest that this probably isn't the problem.
If you are trying to minimize invasive procedures, I'd recommend a few things:
Repeat the bloodwork- it was done when the problem first started and may not have revealed any significant changes at that time. You may find that things have progressed considerably since then.
An abdominal ultrasound by an experienced ultrasonographer. This will help assess the architecture of the abdominal organs, including the liver and intestinal loops, and rule out the presence of masses or other abnormalities in the bowel loops.
If these things are unremarkable,
A colonoscopy- if your dog is fairly even tempered, they may be able to do this awake. More often than not, mild sedation is required, especially if biopsy samples need to be taken. This procedure will allow visualization of at least part of the large bowel and if abnormalities are found biopsy specimens can be obtained. I wouldn't recommend anything really invasive like exploratory surgery at her age, given the other health problems on board.

Problem #2- The heart murmur
In a medium-sized dog this is likely a mitral valve problem. If your vet does not grade it very high (murmurs are graded 1- 6, 6 being worst), then it may not be a significant health issue at this time. Of course, if you do opt to have an ultrasound done the heart could also be evaluated at that time.

Problem #3- The seizures
This is concerning, particularly since they seem to be coming with more regularity. Episodes, even if brief, that occur more than every 3- 4 works, are cause for evaluation. Seizures usually fall into 4 categories: toxic and idiopathic (unknown cause) can be ruled out because they don't fit the signs and your dog's signalment (age, sex, breed). That leaves metabolic (I'd guess we're in here- liver, kidney, sugar, or electrolyte derangements), or brain tumor/lesion. The latter is quite rare in dogs but happens most in geriatric animals. Typically they have other signs, like behavior changes, propulsive walking or circling, which you don't describe in your girl. Again, repeating the bloodwork may elucidate some problems here.

Problem #4- drinking excessively.
Polydipsia (excessive drinking) can also have many causes, including: kidney disease, Cushing's disease (excessive blood cortisol levels), and diabetes mellitus (excessive blood glucose levels), liver problems, diabetes insipidus (a disorder of water metabolism in which part of the brain does not secrete the hormone which tells the kidneys to re-absorb water, or the kidneys do not respond to the hormone), or cancer (secondary to elevated calcium levels). Since she is having diarrhea, she may need to drink more to stave off dehydration secondary to water loss in the stool. Lasix will also cause excess thirst- if the increased water intake around the time she started the medication and she was just recently discontinued, that may be a factor.

Now that's a whole lot of information- it's not a simple problem and will likely need further testing to figure it out. If you wish to pursue that, start with repeat bloodwork. Most important would be liver, kidney, blood sugar, protein, and electrolyte levels. These things will help your vet rule out liver and kidney disease, diabetes, a protein-losing disorder, and Addison's disease. If something is amiss, it may give the answer or guide further testing. The ultrasound will give you a better idea of what's going on inside than the x-ray is capable of doing. The colonoscopy is last, if you wish to go that far.
I wish I could tell you when it's definitely time. At 16 years old, you are probably grateful for every extra day. What I suggest is that you weigh several things: her appetite, her activity level, her interaction with the family, her "enjoyment" of the things she used to love to do. If those things are significantly affected, then it may not be in her best interest to pursue further diagnostics. If she is, for the most part, like herself and you have the financial means to go forward, it may help you figure out the problem and decide if it's something that is manageable.
I realize I sprung a lot of things on you here- please let me know if anything is unclear or if you have further questions.