Zippa AM Chain Guide

Either — it covers both. It bolts straight to ISCG05 tabs if your frame has them, and the ISCG05 adapter included in the box lets it mount on a threaded BSA bottom bracket if it does not. The one case it cannot cover is a press-fit frame with no ISCG05 tabs: there is nothing to bolt to and no threads for the adapter to grip.

The bottom roller. It sits under the chainring and keeps the chain wrapped, and its position is fixed relative to the backplate, so below 32T the ring is simply too small for the geometry to work. The range is 32T to 38T. If you run a 30T or smaller, the Zippa Lite ISCG05 is the one for you — it is an upper guide only and covers 26T to 36T.

No — the bottom roller is the reason. It has to hold the chain at a fixed distance from the chainring, and an oval ring changes its effective radius as it turns, which the roller cannot follow. If you are set on running oval, look at the upper-guide-only Zippa Lite models instead: with those you simply count the ring as two teeth larger than its stated size when you check the range.

Same AL6061 backplate, same top guide, same bottom roller. The Zippa DH adds a replaceable bash plate underneath and a fully CNC'd AL7075 pulley, which puts it at 175g against the AM's 120g. For trail and enduro the AM already has everything that keeps the chain on; the DH's extra 55g is armour, for bike park and downhill where the chainring itself takes hits.

Yes — that is how it is designed. The top guide, the bottom roller and the hardware are separate replaceable pieces, so a worn roller does not mean buying a whole new chain guide. The roller is the part that goes first, since it is the one the chain runs over under load, so if you ride in mud and grit that is the one worth keeping an eye on.