Nexus questions : cross-team dependencies and PO availabilities

1- Is reshuffling scrum teams in the Nexus a good idea to resolve dependencies ? 

I wouldn't recommend constantly re-shuffling teams but if you have empirical evidence showing that moving some team members around will help create more cross functional teams and eliminate dependencies I think there's merit to it. 

2- Is extending a sprint length to achieve integration a good idea to guarentee the production of an integrated increment ?

What is your current Sprint Length? I'd challenge you to first inspect if the teams are taking on too much that they cannot complete integration or if extending the Sprint is really necessary. Consider the impacts of extending the Sprint on feedback time and incorporating changes in the market. 

3- What is the effect on a team's productivity when adding 2 or 3 more teams ?

I'd assume it will initially go down just like adding team members to a Scrum Team. Those teams need to learn how to collaborate effectively within the Nexus and may not always have the context necessary to be productive. 

4- In Nexus with 5 Scrum teams, how can the Product Owner attend all Sprint Planning events?

That sounds exhausting. What else does your Product Owner due that is crucial to the success of the product that they may struggle with if required to attend 5 planning events. Also, you'd have to work all of those events around their schedule... 

We are implementing Nexus this week actually so I can help with this. First off, go buy the book "The Nexus Framework for Scaling Scrum" by Kurt Bittner; such an amazing resource.

1 - I wouldn't reshuffle constantly, I would encourage getting the proper skills for each team to truly be cross-functional and have the appropriate skills to complete an increment.

2 - So are you asking that you're currently at 2 week sprints and you want to change to 3 weeks kind of extension or are you asking that you want to extend the sprint ad hoc by a couple days to allow for integration? If the former, look for the evidence that would suggests this would be a good idea. If the former, you should remember that Nexus follows the same rules within Scrum. Therefore, since extending a sprint in Scrum is not allowed, it would NOT be allowed in Nexus.

3 - This really depends on the teams and product you're working on. Like Tony mentioned, it will likely go down a bit at the beginning as the teams learn the framework and all but it should get back to the normal stage not long after.

4 - I'd encourage you, again, to buy the book mentioned above. Remember that Nexus Sprint Planning is a 2 part event. Part 1: The PO sets the priority and Nexus Sprint Goal. Appropriate Team Members or the entire nexus can participate in this; we are going with the latter and having all team members participate. Part 2 is where the teams break out to form their plans to meet the Nexus Sprint Goal. The PO is not expected to attend all of the breakout sessions because that is for the Dev teams. The PO just needs to be available to be contacted for any questions by the teams.

1- Is reshuffling scrum teams in the Nexus a good idea to resolve dependencies ? 

Do the Development Teams in the Nexus think "reshuffling" would be a good way to resolve their dependencies? Is it how they would choose to self-organize?

2- Is extending a sprint length to achieve integration a good idea to guarentee the production of an integrated increment ?

Why do you think integration might be guaranteed by Sprint length?

3- What is the effect on a team's productivity when adding 2 or 3 more teams ?

Wouldn't that depend upon whether or not the teams had to integrate their work, and the timescale over which productivity was measured?

4- In Nexus with 5 Scrum teams, how can the Product Owner attend all Sprint Planning events?

When Product Backlog refinement is done well, the risks to Sprint Planning are minimized and well understood. Why would the Product Owner's availability then cause an issue?