I can confirm the following:

1) you can summon yourself as a warlock like you mentioned: summon self, hearth, do whatever, and click before expires if you're outside an instance
2) you can also summon members to anywhere within the instance, as long as everyone is in the instance.

what doesn't work is: you cannot be in an instance and summon, hearth, and hit the summon before it expires. It'll give you an instance error and you'll be stuck.