You really are hung up on this "proof" thing. There is no such thing in science, but feel free to continue asking as many times as you like - perhaps I'll eventually come up with the particular combination of words that will serve to finally convince you of that.
To briefly recap the case Abbott makes, the claim is that it is a new variety of plant. The evidence for same is that A) nobody has observed this plant before, and; B) the plant is very closely related to two known types of plant, one native to the UK, and one known to have been introduced to the UK 300 years ago. However, this plant is incapable of interbreeding with those two other species, so it fits the definition of a separate species. Mash the word "new" together with "species" and you get "new species". Really, all this is in the article, but here it is again, lest we lose sight of it.
Now, you want Abbott to go beyond that and disprove any and all alternate theories about the origins of the plant. Well, that's not exactly true - you just want him to address what you assert are "conventional" and "reasonable" alternate explanations. But those aren't his explanations - they're yours. He's got his evidence for his theory, so he's met the burden of supporting it. If you have some "conventional" and "reasonable" alternate explanation, it should be easy enough for you to support it.
"No," you say, "we've just heard why it's reasonable for it to be a flying saucer. If you think it's as silly as someone throwing a baseball, you go and prove it!"
At this point, I thought briefly about simply calling this whole set of requirements you lay out silly, and being done with it all. But upon a moment's reflection, it's not quite so silly - let's look at what you are doing. You want to push Abbott into the position of essentially proving a negative thesis, before his explanation is accepted as valid. But since we can't conclusively disprove any of those contentions, you create a standard for Abbott to meet that canot possibly be met, and thereby create a system whereby Abbott's conclusion can never be accepted as correct. Rather convenient, I can't help but note.
Yes, if someone claims that thing that flew past your window is a flying saucer, and presents some sort of evidence to support it, then the requirement for him has been met - perhaps well, perhaps poorly. You may be convinced, or remain unconvinced due to insufficiency of the evidence or non-cogency of the argument, but you don't get to make the poor fellow disprove whatever your pet theory happens to be, along with everyone else's pet theory that it was a bat or an angel or a flaming pumpkin. If you think it was a baseball, open the door, get outside, and see for yourself if it was a baseball - your theory, your burden, your proof. Period.
Well, because British botany had discovered all British plants by 1993. All of us experts agreed that this was the final volume, no more. That's when we concluded there were no new plants missing from the field books. Every plant since then would have to be new. And look here, it's genetically related to another plant, so it must be evolution. Thank you, thank you."
Very nice. How sad that you leave out one of the key pieces of evidence - that one of the plants it is related to is known to have been introduced to Britain a mere 300 years ago. Therefore, whatever happened to it happened since then Whether it was 220 years ago, or yesterday is irrelevant - it can only have happened in the last 300 years. which makes it a bit more that just previously unknown - it's new.
Oh, and the final volume on British plants was just published recently, not 1993. The "New Atlas of the British and Irish Flora" has descriptions of every single species of flowering plant and fern in the UK. All of them. Which should make it rather harder for the next chap to try to hide behind putative ignorance and claim that there's no way to know if it's really new or has just been hiding.