The C in ECMP stands for “cost”, which is usually calculated as the number of hops or intermediate nodes to the destination. If you were to use link speed as the “cost”, links of unequal speed wouldn’t be equal cost anymore. You can certainly use ECMP over links of different speeds, but as you said bandwidth doesn’t scale for a single connection, so any TCP connections that end up on the slower link perform at the speed of the slower link and it’s difficult to figure out why.

This is intentional: TCP treats packets received out-of-order as representative of an overloaded network. There’s generally a guarantee that packets sent in-order over a single link will be received in that order, but no such guarantee exists for packets sent through two completely independent links/networks. You need something on the receiving end (either at the destination or at the point where the separate paths converge) that will coalesce and re-order packets, such as an MPTCP or PPP implementation.