After more extensive evaluation here is what I conclude:
1.) I agree the site is slow. We are working to improve this, however:the summary card for the site works perfectly. So do others ( I will explain)
2.) I just installed the plug in so the revision was second to last...however; to be certain I updated this morning.
3.) Robot.txt was updated straight away per instructions as follows:
4.) I do not know why you think I am using WP SEO: I am not.
Therefore: following and double checking everything and experimenting on our sandbox (test) site here is what I see
1.) plug in works as advertised on posts and pages.
2.) alternate image can be set using External URL for twitter:image? field.
3.) card type can be changed (to photo for example) alternate image can be set, etc...
HOWEVER: The same does not work on an image post. When editing an image -->>> Media Library --->> Edit (Image) leading to the image post the plugin fields are present but the image always defaults to whatever is in the plugin settings and the field can not be changed.
So, twitter cards for media posts are really not manageable...the description field will update but the image will always default to what is in settings.
So, also, when you say "If you edit a media, it's an attached post and the card can be set but on a different URL. This URL is the URL of the attached post not the parent post which keeps its own "
I understand this. And the parent post is working perfectly as advertised. and yes a card is set on the media "attached post" but image is not able to be changed and is default to what is on settings page.
Thanks for your help. Rich