Arbitrage seems to undo the cascade effect of the small exchange float, with arbitrageurs profiting from that market inefficiency.
That is, ...he actions of the 10% result in the portfolios of the 90% changing anyway, against their will.
...the 90% ...will react by bringing their portfolios back in line. That means they will do the opposite to what the 10% are doing, but with 9x the force: sell their allocations in the minority ledger, now for a giant premium, and buy more BTC at these cheap prices. I believe this negates the "small float" issues
That dynamic requires that one can (and is willing to!) short the small chain. You can not sell what you do not have and can not borrow.
I would also caution against a false premise of external flow. There is no triangular arbitrage outside. Direct flows between fiat and non-bitcoin crypto are negligible. First order, all flows into and out of alternative chains are fully encapsulated in bitcoin, and thus leave the debt/bitcoin ratio unchanged. All the triangular arbitrage is denominated in crypto.