I'll take them in order.
1) The two preferred options (to prevent commercial looting by people who sell collections of scores on eBay without any return to owners) are:
* Creative Commons Attribution Non-Commercial No Derivatives 3.0 (cc-nc-nd)
* Performance Restricted Attribution Non-commercial No Derivatives 1.0
(an IMSLP special)
If Mark had been affiliated with the Performing Rights Society in the UK you would go for the latter licence in order to collect royalties from performances/broadcasts; the CC licences are anathema to the collectors of performance royalties.
2) I think the Copyright remains “© (the estate of) MES” however, given your position as musical executor you could legally qualify that as “© (the estate of) MES c/o DF
3) e e cummings is still in ©, so you would probably have to find out from Mark’s paper work whether he had permission to publish the works in addition to setting the texts. My assumption is that Mark would have had to accept an agreement to return some percentage of the proceeds from sale of the cummings setting to the author (or now, his estate).
As for the Vancouver Songbook: I don’t know which poets contributed to that, but similar arguments may apply there. The worst case is that you would have to reapply to the authors’ estates, because I doubt they would bother to examine their paperwork to see whether the extra allowances for publication had been made: this would mean they have little reason to not charge an arm and a leg for the privilege.
4) It might be an idea to do something like that, just to be all proper and official about it. Feel free to send me a scan of the executorship document if you can easily take a scan or a snap of it.
5) It will be nice to have another, larger setting of Jabberwocky!
More will probably occur to other admins.
PS Looking at the worklist on Wikipedia
, there will be one other © issue with the setting of Robert Graves (†1985) in opus 18, unless it’s a really early poem (pre–1923), which could mean it might be uploadable to IMSLP·US (but not by you!).