I assume that you have already purchased the ticket, otherwise the MPM and CAI starting points are no longer cheap, if available. As you know, do not make any changes until that 1st flight is completed. Forget the RTW tool. With all the recent changes, it is giving errors when it should not and allowing invalid routings. It is causing confusion.

I also assume that you have a stopover in HEL. Why not wait until then to redo you routing. If you need more time, just change the dates of the existing routing once in HEL. Doing a transit thru LHR (not a stop) adds about $50 in taxes.