I would argue they look more like Melon Arms than Jinba, especially in how the shoulders look. The triangular shape at the chest definitely isn't Jinba styled either, I almost wanted to say it's the same as Melon Arms' chest but I can't find any of the placeholder art used early on for comparison (since proportions in the art differ slightly from suit proportions). Still, the shape of the shoulder armor is unmistakably Zangetsu-y
This is the closest thing to toyguide art of Zangetsu, a CG model:
The other picture in the toyguide seems to be a picture of the live action suit.
Anyway, addressing your points... Regarding the chest, I hadn't actually thought about them being triangular. The way I'm looking at it, the risen "pointy" bits are like the ones on the
back of Jimba Arms. The ones in the actual suit get to around that level, although the shape is different. Zangetsu's front armor has a different shape and goes lower than that.
http://www.tv-asahi.co.jp/gaimu/rider/gaim_top.html
http://www.tv-asahi.co.jp/gaimu/rider/
They seem way too high to be in front without just looking silly too. The back just seems like it'd make more sense.
For the shoulders, I guess they'd be a similar concept to Zangetsu's (opening up with a pointy part upwards), but I don't think the mold matches either Zangetsu's CG or the live action suit. Rather than that, the first part seems to match the one in the Jimba-like silhouette there perfectly, while the second closed side is replaced by the Zangetsu-like open spike, which doesn't seem shaped like Zangetsu's anyway, so it's likely a new piece made for the movie Riders.
Also, like mentioned before, their helmets all seem to have Gaim's "wings", and one of them even Gaim's visor. They're probably all based on Jimba Gaim's suit but with modifications. The similarities to Zangetsu is just a similar idea for the shoulder piece "opening". Of course, they're silhouettes at this point, so I could be wrong, especially now that you've raised that doubt regarding the pointy bits that I had immediately assumed were actually on their back. Still, I don't think the Zangetsu idea makes sense. If the Arms aren't modified Jimba ones, they're likely original, although the base suit used still seems to be Gaim's anyway.