Privilege separation set gid program - raspberry-pi

Question

To come to my point, i need to explain the context:
I got a daemon process that opens a posix mq for communication. Clients are in the same group like that daemon to communicate with it. The clients also opens posix mq's and subscribe to the daemon. To be able to communicate, the client mq's must have the same group that the daemon can answer to them.
So far so good, i set the client set gid (chmod g+s client). On a Qt based Desktop (LXQT), the client starts and works as expected. On gtk+ based Desktop (Name LXDE on raspberry pi), it fails to start as gtk+ prevents a set uid/gid programs to use it's library.
As a result i extracted the creation of the mq_open() to a external executable that is set gid (chmod g+s) and uses setegid() to the saved set gid.
The client creates a socketpair(), fork(), execve() and sends the fd through the socketpair (AF_UNIX/SOCK_STREAM) to the client.
The requirements i need to fullfill:
mq's must be readable from all members of the mqclients group
Rights must be 0660 on the mq's
avoid set gid(chmod g+s) on the client
Keep the possible security impact of chmod g+sas small as possible.
Now my point/questions:
I would like to avoid to handle SIGCHLD and kill(), wait() for the mq-opener in the client and daemon. I would just like to readmsg() on the socketpair() and get an error if the mq-opener dies for whatever reason. Just write no signal handler for SIGCHLD?
The fork() procedure and connection with the mq-opener is pretty big. Is there a more simple way to do this?
Can the mq-opener (which starts as a unprivileged user) do the double fork() and drop its parent/child connection to the parent? At what point drops the parent/child relationship?
Would it be better to create a mq-opener daemon that just handle the creation of the mq`s?
To make it a little more clear: Here a diagram:
+-----------+ +------------+
|Daemon | |Client |
+-----------+ +------------+
|File | |File |
|User | |User |
|mqdaemon | |pi |
| | | |
|Group | |Group |
|mqdaemon | |pi |
| | | |
|Rights | +------------+ |Rights |
|a-s | |mq-opener | |a-s |
+-----------+ +------------+ +------------+
|Process | |File | |Process |
|User | |User | |User |
|mqdaemon | |mq-opener | |pi |
| | | | | |
|Group | |Group | |Group |
|mqclients | |mqclients | |pi |
+----+------+ | | +--+---------+
^ | |Rights | | ^
| | |g+s | | |
| | | | | |
| | +------------+ | |
| | fork() |Process | fork() | |
| +------------>|User |<---------+ |
| |(forked) | |
| send_fd() | | send_fd() |
+---------------+|Group |+------------+
|mqclients |
+------------+