How do I call a user-defined command from a function, and wait until it finishes?

Question

The two functions in the script below can be called independently via key mappings <leader>1 and <leader>2. In a perfect world, <leader>1 will always be called first. If the user forgets to do that, I want s:RunMe2nd() to be helpful and call s:RunMe1st(). However, it needs to use the user-defined command to display the legal values and get the one that will be passed to the function.

After some tinkering, I was able to do that with call feedkeys(":RunMe1st \<C-Z>", "t"), but also I would like the execution of s:RunMe2nd() to be blocked until :RunMe1st finishes. Is that possible, and if so, how would I do it?

To see what I'm talking about, source the script, and then type <leader>2. You will see the pop-up menu for the completion items, but underneath it you'll also see an error saying b:init is undefined. echomsg doesn't wait for feedkeys to finish. Source the file again, and type <leader>1 then <leader>2 to see how it works when following the rules.

unlet! b:init   " Re-source this file to reset.

command! -buffer -nargs=1 -complete=customlist,<SID>LegalValues RunMe1st :call <SID>RunMe1st("<args>")
setlocal wildcharm=<C-Z>   " Used to open completion menu.
nnoremap <leader>1 :RunMe1st<space><C-Z>
nnoremap <leader>2 :call <SID>RunMe2nd()<CR>

function! s:LegalValues(ArgLead, CmdLine, CursorPos)
    return ["hello", "world", "42"]
endfunction

function! s:RunMe1st(arg)
    let b:init = a:arg
endfunction

function! s:RunMe2nd()
    if !exists("b:init")
        echomsg "You didn't run RunMe1st first. Let me help you."

        " How do I get Vim to wait for the next statement to finish?
        call feedkeys(":RunMe1st \<C-Z>", "t")
    endif
    echomsg "All good. You chose ".b:init
endfunction

Answer 1

Here is the solution I came up with after reading the link in @D. Ben Knoble's comment. The only thing I'm not terribly happy with is the expand('<SID>') workaround I used. It just seems like a bit of a hack, but it prevents the errors:

• E120: Using <SID> not in a script context: s:LegalValues or
• E120: Using <SID> not in a script context: <SID>LegalValues

unlet! b:init   " Re-source this file to reset.

command! -buffer -nargs=1 -complete=customlist,<SID>LegalValues RunMe1st :call <SID>RunMe1st("<args>")
setlocal wildcharm=<C-Z>   " Used to open completion menu.
nnoremap <leader>1 :RunMe1st<space><C-Z>
nnoremap <leader>2 :call <SID>RunMe2nd()<CR>

function! s:LegalValues(ArgLead, CmdLine, CursorPos)
    return ["hello", "world", "42"]
endfunction

function! s:RunMe1st(arg)
    let b:init = a:arg
endfunction

function! s:RunMe2nd()
    if !exists("b:init")
        echomsg "You didn't run RunMe1st first. Let me help you."
        let answer = input(':RunMe1st ',"\<C-Z>",'customlist,'.expand('<SID>').'LegalValues')
        call s:RunMe1st(answer)
    endif
    echomsg "All good. You chose ".b:init
endfunction