Disco 2 TD5 Auto 2.5 - Lost reverse

A few months ago my LR started struggling with hills and going into limp mode.
I was advised a simple gearbox oil change might help and was easy to do, so I ordered the oil and pump and got to it.

I changed the oil with no problem only to discover that the oil I had ordered was NOT the oil that got delivered (yes, stupid mistake, I know). I immediately drained the oil and replaced it with the oil that was intended to be used.

This was done WITHOUT the engine on and someone flipping through the gears as initially I had not been told this step (I'm an idiot with mechanics).

This resulted in reverse gear disappearing.

The rest of the gears worked fine and the car pulled great.

In an effort to fix the loss of reverse gear, I had a mechanically minded friend come and do another gearbox oil change while I filtered through the gears. I still have no reverse. My friend suggested that it may be a problem with the XYZ switch.

I have had a mobile mechanic come out and check the codes and give me his opinion. There were two injectors showing a code and two codes for a clutch problem that he had no idea about.

His suggestion was that while performing the oil changes there are two places that look like oil drain holes, one is the drain hole and one is something else. If the "Something else" one was opened then a plate inside the gearbox may have slipped down and it would cost £800 for a rebuild.
I don't think any other holes were unscrewed other than the hold to drain the oil and the hole to fill it up.

The lack of reverse is my current major problem.
Any ideas?

Do you see the R on the dash and the reverse lights are working when you select reverse? if yes then your problem might be dscribed below:

R is on the dash but the reverse lights aren't working (according to the missus). Though that could just be a bulb problem I didn't know about.

theres no bolts or bungs you can undo you that could cause a problem ,drain is obvious in the middle os sump bottom, the caps on pump housing if undone would drain the pump not allow anything to become displaced

Seems strange that the problem occured after the oil change.
I'm glad theres nothing that could be displaced. Still clueless as to the problem.