Am I launching this program correctly?

I'm not familiar with pis so I can't say anything definitive on that but I have some thoughts to consider.

--attach_project:
You need to attach only once. Usually you'll use boinccmd for further control but a remote BOINC Manager would be more convenient. Currently your account shows four devices attached, running Debian or Raspbian.

rc.local/cron:
Doesn't your system autostart boinc? At least a standard Debian installation should. I suspect you could have two instances of boinc running, sharing the data directory. That's asking for trouble.

modem/router:
That can't matter here. When you ssh to your dedicated cruncher you're already past the router.

--allow_remote_gui_rpc:
Not necessary for remote control from configured addresses and as it is you're not controlling boinc remotely anyway.

So something was started automatically. The client should still be running. It may not be obvious in (h)top as the client does little by itself, systemctl could tell more:

$ systemctl status boinc-client.service
● boinc-client.service - Berkeley Open Infrastructure Network Computing Client
Loaded: loaded (/lib/systemd/system/boinc-client.service; enabled; vendor preset: enabled)
Active: active (running) since Sat 2020-03-28 14:29:51 CET; 5 days ago
Docs: man:boinc(1)
Main PID: 1034 (boinc)
Tasks: 36 (limit: 4915)
Memory: 11.0G
CGroup: /system.slice/boinc-client.service
├─ 1034 /usr/bin/boinc
├─ 2202 ../../projects/www.worldcommunitygrid.org/wcgrid_mcm1_map_7.43_x86_64-pc-linux-gnu

(This output is truncated where the actual science applications start.)

There comes my multiple instances theory again. Could it be you misinterpreted the message and the default port has already been taken?

The system's BOINC client runs under the boinc user account. When started from rc.local or cron the client runs under the root account and started from the command line it runs under your own account. Originally I assumed all those clients shared a data directory but now it seems they are independent, which is not as bad as I feared but not good either.

That's the system client you want to use. The client is kind of a boss program, if properly configured it makes others do the work. You control it using the boinccmd tool. (Or by editing config files but that's not necessary for basic operation.) Do not run /usr/bin/boinc - or short boinc - yourself as that will (try to) start another client.

So my suggested course of action is: Reboot, configure the system client and don't start any other. This will leave any work already assigned to those clients undone but that's not much at this time.

If you can't get the system client working, plan B is to reinstall it. But make sure to purge it first, that will remove any remains of the existing installation.

A misunderstanding. The argument to the --passwd option is the client's password and by default the client has no password set so whatever you provide there is incorrect.