Right, not a simple problem then! Let me just recap to see if I've got this correct:
Walter Collins, you know for certain, married in the US in 1861, stating a birth year of 1839 and gave his birthplace as Greenock.
The only Walter Collins you can find that fits that is the one in the 1841 census, who is not with his parents. Absolutely no guarantee that is him, but Walter Collins isn't a name that comes up regularly in that part of Scotland, so it's a fair bet.
No birth in SP comes up for Walter in 1839.
I'm wondering how he got to the States, and who with. If he came over on his own volition, he would presumably need to be older than 12, so should have still been in Scotland in 1851, but can't be seen. (I did have a quick look at the 1851 census and like you found Malcolm, or someone that looks to be be him. No sign of Walter, as you say)
If, on the other hand, Walter emigrated before 1851 at the age of 12 or younger, then I presume he he must have gone with somebody. Who? Malcolm McNeil looks to still be in Scotland.
What I'm wondering, and this is just conjecture: was there a scheme from Greenock to assist passages for orphans at that time? I've not heard of one, but no reason why I should. If any of the Scottish experts read this, perhaps they could answer that?
Second, - could he have been a grandson of Janet McNeil? Child of a daughter of Janet's married to a Collins? Might be worth a search on Scotland's People for a marriage
Have you registered with Scotland's People? It's a pay per view site but if any records do exist that answer your question about Walter, it's the best place to start