Tried this as best as I could, using del Capo and King's Ginger + honey. I have never had Montenegro, but I've heard it described as "creamsicle", so I can only comment on the del Capo version. It's a nice cocktail. The combination of lime and dry vermouth is a bit odd, but seems to work here. It's quite accessible, especially given the large amount of amaro.
Dan, I recommend getting your hands on the real deal asap. Calling it "creamsicle" is short-changing it drastically. Without getting too maudlin, I spent my honeymoon on a small island in the Maldives. They made their own essential oil there, which they perfumed the place with. The scent was other-worldly: tropical flowers, ylang ylang, bright sunlight, rainbow auras. Montenegro is the flavor transcription of that scent; it took me right back there. Amazing stuff.
I don't think creamsicle is very accurate. It is gently herbal and orange-flavored, but not one-note, and not very bitter. My best approximation would be half del Capo, half CioCiaro, though Montenegro isn't as spicy as that mix. It isn't on my list of 5 or 10 favorite amari, but do like it. Sibilla on the other hand, well, I just can't quite figure out what to do with that stuff...
For Sibilia, try Krakatoa, August 25th 1883 from the evil mind of Zachary. Also, I use it (Sibilia, not Zachary) in small amounts to perk up level of cocktails that aren't that interesting to me otherwise. Amazing with 1 tsp will do.
OK, I ran out and got a bottle since it has recently appeared in my area of Boston without having to travel to the North End. I'm trying to imagine it as you described, but I get mostly orange, mildly bitter, fairly sweet, and upon the initial sip a brief moment where it comes together as artificial. That does dissipate. By contrast, del Capo was also orange and bitter, but with more alcohol to balance it, and less sweet. del Capo does have some caramel in it, which I would not necessarily say is a good thing, but overall I would rather sip del Capo. I suspect that Montenegro will work as a cocktail ingredient, and I look forward to trying it in this one. In this regard, it seems similar to Aperol; I like it as an ingredient, but not by itself.