TYPO3 Core

I can't reproduce what you described (with TYPO3 3.8.0dev).
one thing I don't understand: what means " take information from the setup field of the mountpoint" which setup field ? the table "pages" has no setup field.

Solving only a part of the problem:

Add to the Setup of either page A or page B (or both, i don´t remeber ;-)
config.linkVars=MP
This will add the needed MP-var to your url for browsing in remote trees:
Every Link will look like this: index.php?id=5&MP=34-89

Behavior: If you load a pagetree B from another part of your Website with all its subpages to page A...

|
|---page
|    |---page B (mounted page tree from here)
|    |       |---page C
|    |       |       |---page D

So, if you come over page A, your menu will look like this in FE:

|---page A
|       |---page C
|       |       |---page D

If you come directly over page B, you will see this in FE:

|---page B
|       |---page C
|       |       |---page D

Every pagetree may have its own setup and layout, so you can reuse a pagetree from two locations with two different setups (like symlinks in Linux).

The problem: If there´s a "shortcut" page in line, for example page C is a shortcut to page D, you will not stay in the setup (and layout) of page A, even if you come from that page. The shortcut is hard and changes the rootline to the mounted pagetree, so you will definetly get a real jump to page D with all the setup (and layout ) of the above pagetree (inherited from page B).

My question is: Why is a shortcut unable to take the MP-var further to its target page? A bug?

Confirmed

That's rather hard to solve as a shortcut is set to a specific page id. I can imagine a solution for the shortcut type "first subpage", everything else is not easily possible IMHO.

Any further ideas on this?

I had a special directory of shortcut-pages which i need to work with mountpoints. As typolink does the job well, i added doNotLinkIt = 1 and creat the link via typolink.

lib.directory = HMENU
lib.directory {
special = directory
special.value = 123
1 = TMENU
1 {
NO = 1
wrap = <ul>|</ul>
NO.allWrap = <li>|</li>

NO.doNotLinkIt = 1
    NO.stdWrap.typolink.parameter.field = shortcut 
  }
}

Can confirm this:

Mounting to a page which is a shortcut to another page (even "first subpage") fails with an error message. If I remove the shortcut - it works.

As we had this "support feature" earlier in the Mantis Bugtracker I will introduce this here in Redmine again and so I will sponsor this issue with 100 EUR if some finds a solution to get MountPoints working with shortcut pages ("first subpage" is enough).

Does the problem is fixed by the patch of #29584 ?