I'm sorry but I don't understand what do you want to acomplish on the last part of your answer 
I must insist on trigering events from the checkbox, I mean, if you check the checkbox you also force a roundtrip(like wen the event is triggered from the dropdown selection) so the value should be stored in the BOL layer, once you acomplish this, try if changing the dropdown works.
If you are saying: "ok, we tried this, we saw via debugger the event is getting trigered once I mark/unmark the checkbox, and the value is correctly updated in the BOL, but when the dropdwon event is triggered I doesn't work again" then I'm afraid you should raise a SAP incident.