It’s random, I think.
Proof of that?
Well, for Legendary mercs, you are guaranteed at least some coins from Heroic bounties of level 30. With the latest batch, it’s actually the Rare mercs that are the hardest to max this way after doing tasks (and previously, some of the epic ones were).
No, thank you. As I’ve said elsewhere, I’ve declined trade offers. As I’ve also noted in that topic, ‘I prefer Visitor tasks, since they can be fun or challenge me.’ Sending that Rattlegore away with its trade offers, instead of having a potentially interesting Visitor task, again and again is less fun for me.
As for other suggestions — sounds more or less fine by me.
One more thing: I generally advise against using Renown on maxing a merc. As for Rattlegore, that’s really a shame they haven’t added it to the reward pool of some bounty — they haven’t forgotten to add Rattlegore coins to she shop, though. 
So, my recommendation would be to not fall into that Renown (or, well… Runestones, wink-wink, nudge-nudge… you know what I mean) ‘trap’ and just wait a bit, until the situation becomes more clear. After all, that ghoul is not running away anywhere — unless you really wanna play it right now, there’s no hurry.