No he's not, he's 5lbs below Oscars Brother in Britain, for a 3 length defeat over 3m. Oscars Brother has been given 155 for the National, 4lbs higher than in Ireland, Koktail Divin got 1lb! Oscars Brother is also only 8-10/1 shot for the BANC, and if they met in a handicap off them weights I'd certainly not be having Koktail Divin as a rag. All the other ratings you note are for Ireland, this is a handicap in Britain, the likelihood is not all of them would be left as they are, so they'd have to give him even more weight.
I also think Koktail Divin is a horse for the intermediate trip, not 3m, and if he was getting the weight in a handicap against the lot above over that trip I'd think he'd have a favourites chance as well, hence I think he is well handicapped. Off levels, maybe not, but then how many of them in the JR Novices Handicap Chase would? None would be my guess, which is why they are going for the handicap.
I disagree about A Plus Tard being campaigned as a handicapper, he won his beginners comfortably, beating the eventual Arkle winner (piss poor though it may be), then finished close up in a Grade 3, this all over the intermediate trip. Koktail Divin, whilst not in the same order, has done similar. Close up in a Grade 2 over a trip too far (potentially), then pissed up in his beginners. You don't have to agree, I don't expect you to, but they are of a similar mould, IMO.
I've backed Sixmilebridge too, but Regent's Stroll is the bet for me outside of Koktail Divin, as he is definitely well weighted if taking literal hurdles form into account, as he was ahead of Koktail Divin at Aintree in the Grade 1 that Honesty Policy won, that was off levels for and a 5 length difference, yet Koktail has to give RS 5lbs now too. The caveat to that is I think Koktail Divin has looked the better chaser so far, but that's not hard looking at what RS has done

I also appreciate this may be by design, and it may look as though Koktail Divin hasn't been ridden with a handicap in mind all season, but I just think Henry is maybe a little bit more honest than some trainers, not all, but some!