Plural with only

Hi,

Is it ok to have plural when using only: heaven is the only place where you can have peace. Suppose we have plural, can we say: heavens are the only places ... peace. (can 'only' be used with a plural noun)?

2) He had cracked and thereby divulged the secret (is this okay starting with past perfect)?

3) Can one say: all of his doubts instead of 'all his doubts'? IS there any difference?

4) Which preposition is right - to or at? I find myself nodding (to or at) his proposal.

Thanks.