i could buy the ten -speed bicycle myself,but my mother promised to give it to me on my birthday.

绝对正确!
1.改正：I (could have bought) the ten-speed bicycle myself,but my mother promised to give it to me on my birthday.
从后句的时态,可以看出前句应该使用虚拟语气：could have done sth,它表示“本可以/能够做某事”,隐含“但是因为某种原因,当时（过去）没能做到”意思.
2.答案：had been sent
此题明显使用了(even) if引导的条件状语从句的虚拟语气.
从句意可知是对过去动作的虚拟,所以主句用了would (not) have done sth,而从句就必须使用过去完成时态（且是被动语态）即had been sent.