Wouldn't Ubuntu Mate 16.04.2 connect to the same repository as Ubuntu Mate 18.04.2 anyway?
I don't know what i said apply to flavored/modified Ubuntu, but AFAIK :
1. it connects to same repository, but
2. Each Ubuntu version/release got different application version
3. LTS version prefer compatibility and stability. After few years, you won't able to get latest version of application.
If you want get latest version of all application, you must use newest version and update your Ubuntu regularly.
For the sake of learning, how should I have entered
sudo install -m 0755 -o root -g root -t /usr/bin ~/.bitcoin/bitcoin-0.17.1/bin/*
minus the root priledges?
Practically, what risks did doing this introduce? (Given that the Raspberry Pi and 1tb drive are single-purpose, meaning if all the data on micro sd card and 1tb drive were damaged, nothing irreplaceable would be lost.)
I rarely use
install command, but AFAIK for install/modify application, you need
sudoers ability.
I don't know about possible risks (besides you install malicious application to
root user), but if you wary about it, you should install it to user/group who don't have
sudoers privilege.
Personally i prefer use
dpkg -i package_name.deb command or simply run an application with syntax
./bitcoin-0.17.1/bin/bitcoin-qt.