Dealing with irresponsiveness from far-away scientists
First: that situation is common. Actually, that situation is very common. You listed a large number of options that come to mind, let's discuss them a bit:

For me, that's the best.

Reminders don't cost you much, but they can only go so far before you risk being annoying.

I would strongly advise phone or Skype. Don't get me wrong: I hate phone too. It has the inconveniences of both verbal communication (you need to reply fast, and have no time to smith your words precisely) and written communication (you don't see the other person's face and non-verbal indicators). However, it helps if people are unresponsive to email. Also, consider that maybe they like phone better than email (some people do!).
As I said, Skype (or any video-chatting tool) might be a useful alternative to phone. Some people hate it, some people love it, so be sure to ask if they would consider it.

No. You can ask a friend or common acquaintance to poke them, but don't use someone you don't know well. Unless it's their job, of course: if you communicate with a big boss or dean or whatever, it is fine to use his personal assistant or secretary to check up on things or remind him of deadlines.

and 6. It's your choice to make, not much we can say here.



To your (already good ideas) I would add an important one:
   7. Set yourselves (or give him) a goal and a deadline. On one occasion where you actually meet him, use his enthusiasm to set an (achievable) goal, possibly with an associated deadline: decide that you want to present this work at this occasion, or submit to a given special issue that has a deadline in 6 months, or want to have the project finished by September to hire a post-doc with funding from the John Doe Foundation, …
Whatever the deadline is, it will spur contributions from his side. The research environment puts people under a lot of pressure with bureaucracy and deadlines. If your project doesn't have any visible deadline, he will never get it done. So, create one, even if it's more of a pretext. Then use that deadline as a hammer!