What is the correct way to setup apt pinning in debian?

Question

linux

Purpose: so that the installation of packages obeys the rules I defined and during installation it was not necessary to specify where to install.
Rules (approximate):

• If the package is in jessie backports, then install from jessie backports
  
• If there is no package in jessie backports, then install from standard repositories
  
• With pkg.tox.chat install the qtox package and some of its dependencies, regardless of whether they are in debian's native repositories
  
• Do not install any more packages with pkg.tox.chat
  

Contents of /etc/apt/preferences:

Package: qtox libavformat-tox56 libavdevice-tox56 libavcodec-tox56 libavutil-tox54 libswscale-tox3
Pin: origin "pkg.tox.chat"
Pin-Priority: 991

Package: *
Pin: origin "pkg.tox.chat"
Pin-Priority: -10

Package: *
Pin: release n=jessie
Pin-Priority: 500

Package: *
Pin: release n=jessie-backports
Pin-Priority: 999

Package: *
Pin: release n=jessie-updates
Pin-Priority: 500

Questions:

1. Did I do everything right?
   
2. Will ordinary system/security updates from security.debian.org fall off?
   
3. Is there something I'm missing and is there a smarter way to do it?
   
4. Does higher priority (within the same range described in man apt_preferences) mean higher source priority? For example, a package exists in two repositories. One has a priority of 991, the second has a priority of 992. Will the package be installed from a repository with a priority of 992?
   

Answer 1

Regarding qTox, this is not the problem, there, after the community separated and climbed to a new site, the repository maintainer has no time to fix the repositories for new OS versions, I have a similar situation in Ubuntu 15.10, since I usually update packages through Synaptic, it’s more convenient with test versions of Ubuntu - then fixed the packages there.
Issue in the repository, you can comment/subscribe: https://github.com/irungentoo/toxcore/issues/1402

Answer 2

If the question is related only to qTox, then I didn’t do pinning at one time, so as not to get confused later on what came from where, but collected qTox statically for different distributions and put it in a publicly accessible repository - now I just update the sources once a week and then in a regular way deb arrives.
If anyone is interested, then tox.pkg . In particular, the repositories for qTox .