It is indeed true that there is no such thing as a 100% MS diagnosis, except by post mortem, which I’m sure is not the way most of us hope to go about it!
However, there are established diagnostic criteria which are treated as sufficient, in practice, for diagnosis, because obviously you do not want to have to wait for the patient to die before you can say what was wrong with them.
I don’t know what the position is in Spain, but in the UK, most neurologists still want lumbar puncture results to help confirm diagnosis, although NICE (the official organisation that sets out standards for diagnosis and care of patients here in the UK) make clear this is not essential for diagnosis, and in fact, being an invasive procedure, should be the exception rather than the rule, and only used where other evidence is ambiguous. I am diagnosed without a lumbar puncture (I refused to have one!) The diagnosis is perfectly legitimate, as I had clear symptoms and very typical evidence on MRI - plus I was tested for over 100 other things, all of which were negative - so further evidence was not really needed. It wasn’t a massive leap of faith to say: “If it looks like a duck, walks like a duck, quacks like a duck - all tests for ‘non-duckness’ have come back negative - then we can call it a duck”.
I think, if you want to be pedantic about it, any diagnosis of anything has a certain margin of error - mistakes do happen. MS is a difficult diagnosis, because there is no one specific test, so perhaps the margin is higher than for some other conditions. But it’s not high enough to make diagnosis impossible. It just means they like to gather as much evidence as they possibly can before committing.
I hope you don’t mind me saying, but in my opinion it may have been a mistake to turn down the steroids. There is no reason in principle why symptoms cannot be treated in advance of a definitive diagnosis. You do not always need to know the name of a disease before you can treat the effects. If it’s “highly probable” you have MS (and you yourself believe this), then it’s highly probable steroids would have been of some assistance.
“Acute” is simply the opposite of chronic. It has nothing to do with how far you can walk, or how long it takes you to recover afterwards. It just means you haven’t had these symptoms all the time, or for years and years, but there has been some sort of episode, or flare-up. Steroids are not usually offered for chronic (long-term) symptoms, because they have not been shown to be effective for that, but they can be effective for these short-term flare-ups (also known as relapses), helping to speed recovery.
Having said that, it is ALL they do. No study has found them to alter the extent of recovery, or the long-term course of the disease. In other words, if you refuse steroids, you will get back to exactly the same point you would have done if you’d had them - just more slowly. They help natural recovery, but don’t improve on it, except in terms of speed. So you don’t have to worry you’ve caused yourself lasting damage by saying no. You may have been putting up with things a little longer than absolutely necessary, but there will be no long-term consequences. Steroids have their own risks and side-effects anyway - so not all neurologists (or patients!) think they are worth it, given that they have no effect on the long-term prognosis. It depends how bad the acute episode (relapse) is, really. I have always said that if I couldn’t walk at all, or couldn’t see, I would have to have steroids, as I’d want to recover as fast as possible. However, if symptoms are frustrating but livable with, I’m patient enough to let nature do the work.
I find it interesting that you didn’t mind taking Baclofen, despite still not knowing officially what’s wrong with you, yet felt it would be “wrong” in the case of steroids. Although they are completely different classes of drugs, both are really just about trying to control symptoms as far as possible, and not about fighting the underlying disease. So if you were willing to accept one, why not the other?
Apart from steroids, there’s really not much that can be done about an acute episode (relapse), so it’s either that or wait it out the natural way. Sorry!