Wanted to get a bit of combined wisdom before I try with BA or Propeller travel for a more complex GUV situation.

I want to do a multi-city booking and use a GUV... this is basically me doing my 4 BA flights a year (I currently live in Asia), hence a bit more of an inconvenient routing, but one that puts me on BA rather than Lufthansa for a flight to Germany, and as otherwise the GUF will expire. Looking on ITA Matrix the following fare classes are available for a cash seat in business
LHR-KEF (R) // KEF-LHR (R) -FRA (I) // FRA-LHR-MAN (I)

However, similar to flashware's recent post, the LHR-KEF route does have rewards availability even though no I class seats are available.

So, can I ring BA and do the GUV, accepting that my KEF-LHR section can't be in business, but all the rest can (i.e. I am mixing the use of I and redemption buckets)? It seems in this scenario I am better calling BA, do I just call the normal ticketing line, or a redemption one?

Are there going to be other complexities such as does this make it invalid (from the T&Cs)?