Me and my girlfriend just got EC

1) I am unmarried partner of my girlfriend and need to know that do we have to enter to The UK together?
2) As she is HSMP holder how long she must stay in The UK or how long she can stay out of The UK? (she is working for the German company and will join the trainy program for 4 months in Germany)
3) What if the HO officer visits us during this trainy period. How can I explain them that she is in Germany for short term and move back soon and will they accept this explaination.
Best regards,