Get All Functions In A PowerShell Script

The AST is the way to go for static(ish) analysis. Here's how I would do what you described

$rs = [runspacefactory]::CreateRunspace()
$rs.Open()

# Get the AST of the file
$tokens = $errors = $null
$ast = [System.Management.Automation.Language.Parser]::ParseFile(
    'MyScript.ps1',
    [ref]$tokens,
    [ref]$errors)

# Get only function definition ASTs
$functionDefinitions = $ast.FindAll({
    param([System.Management.Automation.Language.Ast] $Ast)

    $Ast -is [System.Management.Automation.Language.FunctionDefinitionAst] -and
    # Class methods have a FunctionDefinitionAst under them as well, but we don't want them.
    ($PSVersionTable.PSVersion.Major -lt 5 -or
    $Ast.Parent -isnot [System.Management.Automation.Language.FunctionMemberAst])

}, $true)

# Add the functions into the runspace
$functionDefinitions | ForEach-Object {
    $rs.SessionStateProxy.InvokeProvider.Item.Set(
        'function:\{0}' -f $_.Name,
        $_.Body.GetScriptBlock()) 
}

# Get help within the runspace.
$ps = [powershell]::Create().AddScript('Get-Help MyFunction')
try {
    $ps.Runspace = $rs
    $ps.Invoke()
} finally {
    $ps.Dispose()
}

You could also use the $tokens from near the top if you want to go a purely static route. The comments won't be in the AST but they will be in the tokens.

Edit The method above actually loses comment help somewhere in the process, not because of the runspace but just because of how the function is assigned. Likely due to the comments not really being a part of the AST. In any case there is a more direct and more static way to obtain the help.

Instead of defining the functions, you can use the GetHelpContent method on FunctionDefinitionAst

$helpContent = $functionDefinitions | ForEach-Object { $_.GetHelpContent() }

This will return a CommentHelpInfo object for each function. It's important to note that this is not the same object returned by the Get-Help cmdlet. Most notably it does not distinguish between things like the code and the description in an example block. However, if you need the CBH to be parsed as normal you can get the comment block text and define your own fake version.

$helpContent = $functionDefinitions | ForEach-Object {

    # Get the plain string comment block from the AST.
    $commentBlock = $_.GetHelpContent().GetCommentBlock()

    # Create a scriptblock that defines a blank version of the
    # function with the CBH. You may lose some parameter info
    # here, if you need that replace param() with
    # $_.Body.ParamBlock.Extent.Text
    $scriptBlock = [scriptblock]::Create(('
    function {0} {{
        {1}
        param()
    }}' -f $_.Name, $commentBlock))

    # Dot source the scriptblock in a different scope so we can
    # get the help content but still not pollute the session.
    & {
        . $scriptBlock

        Get-Help $_.Name
    }
}