I just found information which leads me to this: could Marcomir be a Gaulish* name? I found out that the Gaulish word for horse was 'marka', and that they also had a word 'maru', which meant 'great.' These elements combined, it should be 'Markamaru', which meant 'great horse.'
But Marcomir could be a Germanic name after all - from 'marco' (boundary) and 'mari' (famous), as you said.
Btw, you apparently named 'marha' (horse) as a Germanic element, while the Germanic equivalent of horse actually should be 'hros'. Or did your book also list it as a Gaulish word?
* Gaulish is an extinct Celtic language (it was spoken in Gaul) of which very few records and names survive.