NDrange, work-itme level parallelism vs work-group level parallelism

There is no explicit multi-threading per CU, unless you use SIMD. In other words, without SIMD, there will never be two work-items from the same work-group entering the CU pipeline at the same clock, simply because there is just one pipeline, and the work-items are instead pipelined.

With multiple CUs, however, you can assume that you have some high-level multi-threading, since there will be work-items from different work-groups running in parallel in different CUs.

There is little to no info about the inner workings of Altera's compiler and how the circuit is implemented, other than what exists in the two guides you mentioned. Anything I say here is based on my own understanding of the compiler after experimenting with many kernels over the past few years.