Hi Joy,
I couldn't make any sense of the other possibilities for S15 either. This is why I said your first suggestion was intriguing, as it makes a sort of sense and doesn't fall over on the first hurdle. For the purely geometric scenario where the S7 fibre of S15 gives topological monopoles in space (S3), the physical scale of the compactified S7 dimensions defines the scale with which all measurements in space are made. But since the S4 basespace of the S7 in the S8 basespace of S15 isn't involved in this, the relativism of measurement makes the S4 genuinely hidden as it cannot be measured using anything that exists in space - like trying to measure a vertical distance with a horizontal ruler.
The underlying reason for your characterisation of my S1*S10 and S15, is that in my case the time dimension is left hanging outside of the unification, whereas in the S15 case all dimensions are unified on an equal footing. The price you have to pay for that is to explain the origin of the time dimension, and since time is the basis for dynamics, it has to be explained first. This would give a sequence of symmetry breakings - i.e. breaking of the equivalence of dimensions in S15:
time dimension: S15 = S7*S8 -> S7*(S1*S7)
Higgs vacuum: (S7=S3*S4)*(S1*(S3*S4)) -> (S3*(S3*S1))*(S1*(S3*S4))
Monopoles would appear at the second breaking - if it is valid to map S7 to the S3 fibre of a space only, which it may not be - so it must come last otherwise the topology of S15 would give additional features that would probably conflict with particle physics. The breaking sequence could give different vacuum options prior to the Higgs vacuum, as only one option would be selected in classical physics and the universe would stay there, i.e. no quantum tunnelling between vacuums as in QT.
Best,
Michael