Honestly, the only thing I thought was weird was the idea that an idiot like Harriman would even have a career at that point. A guy whose only on-screen appearance made him seem like a wimpy, incompetent captain was not someone I imagined even retaining his rank after killing Kirk. I always imagined him being demoted to Commander after the Kirk incident and spending the rest of his career as the commander of a small starbase safely in the middle of federation territory. .
Since you mentioned that the only other novel with Demora you read was TCD, I asume you haven't read Serpents Amongst the Ruins, the other novel featuring Demora AND the Enterprise-B, captained by Harriman. Considered by a lot of TrekLit fans as DRGIII's best work, and one of the best Treklit novels, it not shows us a bit of why Harriman acted the way he did in Generations, but also how, in the following years, he became one of the finest captains in the fleet.
Generations is set in 2293, Serpents was set in 2311. This novel is set in 2319. Is it so hard to accept that, just like real people, someone like Harriman could have changed a lot in this 26 years?