To find some common ground re: "does an implementation qualify as
Scheme", how about building a conformance chart from the bottom up?

Split RnRS into a list of features, and for each feature, determine
useful conformance levels.

Something like this:

call/cc - full / escape only / none

syntax-rules - full / unhygienic / none

tail call elimination - full / named let only / self-calls only / none

integers - n-bit fixnums / same as flonums

exact rational numbers - ...

inexact real numbers - IEEE flonums / none

complex numbers - ...

(scheme base) library - all procedure and syntax in R7RS-small / x,y,z
missing

(scheme lazy) library - promises are a disjoint type / promises are
procedures / none

...