It does make a difference if they ranked for those terms first, and you did a 301 redirect to the main site. If not, then it will not really have any effect.

It will only help if those domains have backlinks. Otherwise it won't provide any benefit.

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definitely they will help, in fact they would be much helpful if the content on these site is also relevant.

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Yes. Keyword-rich URLs can have a slight impact in terms of search engine rankings.

If that does not have a high PR will not have any good or bad effect on your main site.

What do you mean by "Pinting to"? Give links to or redirect to? Anyway, those websites' quality would be the key. If they are high quality, it would be beneficial, but there is one thing you need to pay attention to, do link or redirect those websites to the one website at the same time, it might be harmful.

Well, Multiple keywords will never hurt your Indexing but only if they are relevant.