Yes they can, but it doesn't really matter. They can't send you a faked dns response if you are connecting to the website via HTTPS, because a fake website won't have the correct certificate.
They may block your DNS queries so you can't connect to things, but they're your ISP, they can block all of your traffic anyway.
The worst they can do by injecting their own DNS server is to track your queries, but if you're not doing complicated stuff you are telling your ISP which sites you visit anyway (because of TLS Server Name Indication) and where your traffic goes (because the ISP needs a target IP to route your traffic).
A VPN "solves" all of these problems in so far as that now your ISP has a harder time tracking you, but you just moved the problem and now have to trust whoever is running the VPN server.
I'm not sure spotifyd is just spotify (Edit: I checked, its some kind of spotify client meant to be run as a daemon? No idea what permissions that needs)
And the user that executes a service isn't determined by who owns the service file, there is a user option in the service config
Boot from a live disc/usb, check the kernel logs. That should at least tell you where the boot process got stuck, what to do about it depends on what exactly broke.
Yes they can, but it doesn't really matter. They can't send you a faked dns response if you are connecting to the website via HTTPS, because a fake website won't have the correct certificate.
They may block your DNS queries so you can't connect to things, but they're your ISP, they can block all of your traffic anyway.
The worst they can do by injecting their own DNS server is to track your queries, but if you're not doing complicated stuff you are telling your ISP which sites you visit anyway (because of TLS Server Name Indication) and where your traffic goes (because the ISP needs a target IP to route your traffic).
A VPN "solves" all of these problems in so far as that now your ISP has a harder time tracking you, but you just moved the problem and now have to trust whoever is running the VPN server.