Marauder's Set: what counts as a hatred spender or generator?

I have the 4-piece bonus of the Marauder's Set, but not the 6-piece yet. I also have the Spines of Seething Hatred, which means that my Chakram generates hatred, rather than spending it.

I'm having trouble understanding the 4-piece bonus,

Sentries cast your Hatred spender when you do.

It seems to work fine with Multi-shot, so I assume I'm not confusing the meanings of 'spender' and 'generator'; I use Multi-shot and three more spray out from my sentries. But Rapid Fire, with the Bombardment rune, doesn't appear to work (ie. I don't see my sentries dropping a stream of grenades, or even any grenades), whereas it does work with Chakram (I use it with Twin Chakram, and my sentries cover the screen with them). I thought this was pretty weird, because the 6-piece bonus specifically declares Chakram as one of my generators.

So, (a) does this just mean the Chakram is covered by both set bonuses (ie. it's a spender and generator of hatred), and (b) does the 4-piece bonus work with Rapid Fire, and if so, why can't I see it working?

EDIT: This question argues that Chakram would be considered a spender but not a generator. But as far as I can see, I seem to be able to have my cake and eat it too, if the 6-piece bonus text and my experience are both accurate:

Your generators, Elemental Arrow, Chakram, Impale, Multishot, and Cluster Arrow deal 100% increased damage for every active sentry.

I'll also keep this question open because I'm still confused about how Rapid Fire works with this.