Hi @newfast, I've been following this thread with interest... you seem to have gathered up a number of options.
The repair is still within the scope of either D-I-Y or a Menshed project.
Having seen the fuller setting from the photos you sent, I can see that going through the back of the door is not a good option.
The way that the tradie is suggestion sounds like it is similar to what I would also now suggest that you (if you have the tools and desire to), or a Mens-shed do....
Remove the door so you can lay it flat:-
1) Put masking tape on the inner edge of the moulding to protect it from damage. (It will probably need painting, but let's minimise damage.)
2) Using a knife, pushing into the corner of the moulding and panel section to be removed at about 45 degrees, cut around the inner edges (not deep) to cut through the paint and also provide a guide for further cutting here. Then cut around again at 90 degrees to the panel to widen the first cut a little.
3) Using a multi-tool with a saw fitted cut around the edges of the damaged panel right in where it joins the moulding (i.e. where you last cut a groove with the knife). I suggest using the semi-circle saw blade as it self-cleans better and its larger surface area will guide it along the masked moulding without wandering off easily.
I suggest a multi-tool because there isn't enough depth in the door to get another type of saw in and move it in and out. You may have to cut any curved sections through with the knife.
4) Once the panel has been removed and edges cleaned up, you will need to use the removed panel section as a template to cut a new insert out of Masonite (allow for the extra the saw blade removed but it doesn't have to be too tight a fit anyway, else it may bend.)
5) you will also need to pack some timber inside the hole to hold the new panel at the right depth. That would be made into a frame (but doesn't need to be joined at the corners) which you would glue to the back panel (cutting away any honeycomb infill that is in the way if the door has it. It will be easy to cut.). The timber strips may need to be sanded, rasped, filed or whatever, to the right thickness to hold the new panel out correctly.
You can use some dobs of hot-melt glue to keep the strips in place initially. Also put a strip or two across the middle to stop the panel bending inward.
6) Once it all looks like it will go in properly, apply liquid nails along the back edges of the strips to the hold them long term to thr back panel and also glue their outer surfaces, where the panel will touch them. If you have some rubber suction pads, they may help you control the panel while you insert it.
(Dip them in warm water, dry quickly and press them onto the board. Push any air out the sides and they tend to stick ok.)
7) When the glue has dried (overnight?) fill the cut edges (which should be just a small gap) with some fine bog and sand smooth when dry. You may have to go over this twice to get a neat, invisible corner.
😎 Paint. Refit.
