Point 1. This seems to work. Adjust file names and timings as required.

Option Explicit

Sub Macro1()

Dim WMP As Object
Set WMP = CreateObject("new:{6BF52A52-394A-11d3-B153-00C04F79FAA6}")

Const FileName1 As String = "S:\test\abc.mp4"
Const FileName2 As String = "S:\test\abc.mp4"

Application.DisplayAlerts = False

Do

Dim i As Integer
i = i + 1   ' get out of jail; adjust as required
If i > 1 Then Exit Do
    
    Sheets("Sheet1").Select
    Range("A2").Select
    
    Application.Wait Now() + TimeValue("0:00:05")
         
    Sheets("Sheet2").Select
    Range("A2").Select

    Application.Wait Now() + TimeValue("0:00:05")

    Sheets("Sheet3").Select
    Range("A3").Select

    Application.Wait Now() + TimeValue("0:00:05")

    Sheets("Sheet4").Select
    Range("B3").Select
    
    'insert code here to play video (19 sec), video is on S:\test\abc.mp4
    Debug.Print "first"
    WMP.OpenPlayer FileName1

    Application.Wait Now() + TimeValue("0:00:19")
    
    Debug.Print "second"
    WMP.OpenPlayer FileName2

    Application.Wait Now() + TimeValue("0:00:19")
    
    'Shell ("taskkill /f /im wmplayer.exe")
    Shell "CMD /C TASKKILL /IM wmplayer.exe"
    
    Debug.Print "end"

Loop

Application.DisplayAlerts = True
   
End Sub
Point 2. Nothing I have tried works. I Googled this: excel vba make windows media player open full screen. Unfortunately, none of the suggestions helped although I could be missing something.