Just officially finished all we can do on a Stein panel install a few hours ago. Don't know what stage you are at in your build but when the panel and harness showed a few weeks ago the fuselage was just painted and still on the rotisserie. My first time working under the panel with it inverted and what a difference with the install. I've done six or more over the years doing all the harness work myself. this time we had a Stein harness but it still needs tweaking to make things fit properly . Two weeks involved working under there, no cuts, bruises or sore back.
Now to answer your question, yes it can all be done without the top skin off. We just did it , mind you inverted. As with you my friend didn't want all the screws showing that fasten the access panels. Servicing will require being on your back from today forward.