Try to reload like this:

set importlib = ##class(%SYS.Python).Import("importlib")
do importlib.reload(helloWorld)

Also, it not an IRIS-specific behavior, you'll get the same results in any python interpreter:

import helloWorld
helloWorld.helloWorld()
>'Hello world'
del helloWorld

# modify helloWorld.py in text editor

import helloWorld
helloWorld.helloWorld()
>'Hello world'

Check this example.

In short:

1. Create class extending %SYS.Task.Definition
2. Add properties - that's task settings
3. Implement OnTask method, which returns %Status
4. Set TaskName parameter

After that you can add a task of TaskName type from the SMP.

Assuming you have an EnsLib.HL7.Message object, call OutputHTMLZen method for it.

1. Yes. Stopping primary makes backup a new primary. Before stopping you might want to validate that backup is caught up:

set sql = "SELECT CASE WHEN DatabaseLatency=? THEN 1 ELSE 0 END IsCaughtUp FROM SYS.Mirror_MemberStatusList() WHERE CurrentRole = ?"
set rs = ##class(%SQL.Statement).%ExecDirect(,sql, "Caught up", "Backup")
do rs.%Next()
if rs.IsCaughtUp {
	write "Caught up"
	halt
} else {
	write "Not caught up"
	do $system.Process.Terminate(,1)
}

Or at least is not too far behind (check this post).

2.  No. ^MIRROR does the same.

There's a distinction between Background Jobs and Background Tasks.

Backgound job is anything you run using JOB command. Background Task is a limited set of named actions (AuditCopy, AuditExport, AuditPurge, Backup, CompactDBSpace, CopyNamespaceMappings, CreateDatabase, Compile, DatabaseIntegrityCheck, DataMigration, DefragmentDB, Delete, EnableEnsNamespace, Export, ExternalLangServers, FileMan, Import, SQLExport, SQLImport, SQLExportStatement, SQLImportStatement, QueryExport, JournalIntegrityCheck, LinkTable, LinkProcedure, MirrorActivateCatchupDB, MirrorRemoveDB, MirrorMountDB, MirrorAddDatabases, ModifyDatabaseSize, RebuildIndices, TuneTable, TuneTables, PurgeAllCachedQueries, JobShowPlan, JobSaveQuery, JobPossiblePlans, JobComparePlans, ShardActivate, ShardAssign, ShardVerify, ShardRebalance) runnable through a special interface. Docs.

You can run a Background Task (but probably should not  - it's a system action) by calling:

set parms("ClassName") = "Sample.Person"
set tSC = ##class(%CSP.UI.System.BackgroundTask).RunTask("RebuildIndices", "SAMPLES", .parms, .job)

As HIHLib.Support.GetHL7MessageStat:ISBListingQuery is not on the list you can't run it as a Background Task, but you can run it as a Job.

Next - no output. Note that you do not specify these parameters:

principal-input	Principal input device for the process. The default is the null device.
principal-output	Principal output device for the process. The default is the device you specify for principal-input or the null device if neither device is specified. UNIX®: If you do not specify either device, the process uses the default principal device for processes started with the JOB command, which is /dev/null.

Further:

Only one process can own a device at a time. This means that a job executing in a JOB Server is unable to perform input or output to your principal I/O devices even though you may close device 0.

So by default you run your job with stdio set to /dev/null and that's why there's no output.

You can either pass a device to write output to or write a wrapper which would handle the output and job that.

To install locale, execute during docker build:

set sc = ##class(Config.NLS.Locales).Install("deuw")

Check this answer. And this article.