I can just repeat myself: Set Crawl to yes and use rel canonical with website.com/?v3 pointing to website.com

I'm getting a bit lost with your explanation, maybe it would be easier if I saw the urls, but here"s a brief:

I would not use parameters at all. Cleen urls are best for seo, remove everything not needed. You definately don't need an url parameter to indicate that content is unique for 25%of traffic. (I got a little bit lost here: how can a content be unique for just part of your traffic. If it is found elsewhere on your pae it is not unique, if it is not found elswehere, it is unique) So anyway those url parameters do not indicate nothing to google, just stuff your url structure with useles info (for google) so why use them?

I am already using a link rel=canonical statement. I don't want to add this to the robots.txt file as that would prevent the juice from being passed.

I totally don't get this one. You can't add canonical to robots.txt. This is not a robots.txt statement.

To sum up: If you do not want your parametered page to appear in the serps than as I said: Set Crawl to yes! and use rel canonical. This way page will no more apperar in serps, but will be available for readers and will pass link juice.

What do you men by url parameter specifies content?

If a page is not crawled it definately won't pass link juice. Set Crawl to yes and use rel canonical: http://www.youtube.com/watch?v=Cm9onOGTgeM

It is legit as carries user benefit. I'm pretty sure that is forwards link benfits as well.

I think this is not real SEO itself, but rather cross-marketing or brandring issue, however owner of all brands is the same I suppose.

I think this is evertything you can do. Do 301 or implement canonicals from the non slash versions of the urls to your preferred slash versions. There will be only a handful I think who will type in your urls manually rather them copying and pasting them when they want to link to it. You can do nothing with the rest: as you said: ask them to correct or bear that 10%loss of link juice.

I had just the same experience. It was only one occasion but I did nothing more to the site then putting it under a new account on my shared hosting, so only the last digit of the ip has changed. I saw a drop in rankings however the original I gained back the original rankings a few weeks later.

Egol has usually got great answers that woths linstening to, this time however I think I disagree as moving site A's hosting to a new ip causes a drop in rankings. Put the redirection on top of that and you get some more fallback. I think in the above case I would not change the hosting but do the redirect and wait for google to notice the change. Maybe a few months later I would give up site A's original hosting and migrate it to site B's hosting to be able to keep the original urls live for some more time.

And that is even a better advice than mine.

If I get it right sites are based on the same topic, so interlinking them carries both user and search engine value (maybe a little bit less for engines than for users as google maybe knows that you own both sites so it gives less value to the links). When you do your linking just keep in mind what pages would you like to rank with and hw pagerank flows so do your job based on that http://perthseocompany.com.au/seo/tutorials/a-simple-explanation-of-pagerank (thanks ALan). Good luck!

Repeating just a snippet of infromation is not considered to be duplicate content. So if you have a few lines of text appearing repeteadly somewhere on your site, or multiple occurances of a video taht is not a duplicate content as long as the page contains enough individual information.

The other thing is that engines can't really find out video content they just make conclusions based on the descriptions you give them. Vary descriptions and titles.

It has some impact bust not very serious, I would say converging to zero. I think writing a good description that answers the serchers's querry and generates clickthroughs is more important than stuffing your kw in the first few words.

If I get it right sites are based on the same topic, so interlinking them carries both user and search engine value (maybe a little bit less for engines than for users as google maybe knows that you own both sites so it gives less value to the links). When you do your linking just keep in mind what pages would you like to rank with and hw pagerank flows so do your job based on that http://perthseocompany.com.au/seo/tutorials/a-simple-explanation-of-pagerank (thanks ALan). Good luck!

Repeating just a snippet of infromation is not considered to be duplicate content. So if you have a few lines of text appearing repeteadly somewhere on your site, or multiple occurances of a video taht is not a duplicate content as long as the page contains enough individual information.

The other thing is that engines can't really find out video content they just make conclusions based on the descriptions you give them. Vary descriptions and titles.

It is legit as carries user benefit. I'm pretty sure that is forwards link benfits as well.

I think this is not real SEO itself, but rather cross-marketing or brandring issue, however owner of all brands is the same I suppose.

Egol has usually got great answers that woths linstening to, this time however I think I disagree as moving site A's hosting to a new ip causes a drop in rankings. Put the redirection on top of that and you get some more fallback. I think in the above case I would not change the hosting but do the redirect and wait for google to notice the change. Maybe a few months later I would give up site A's original hosting and migrate it to site B's hosting to be able to keep the original urls live for some more time.