How can I reduce creaking sound in 100 year old stairs?

Question

I have a house built in 1928, with two large stair cases (straight to small landing, then curved). When we bought the house they were covered in vinyl, and had plywood nailed in between the rear of the tread, up to the nose of the tread above (so angled / in front of the riser). The back of the stairs is all plastered so I can't easily access it.

I do notice when I walk on it that a lot of the noise appears to come from the riser itself moving when I walk on the tread above once they are attached firmly. However, to be honest, the whole thing sounds like it is squeaking - not just one place.

Then once I removed the vinyl we found this:

After removing the board it was then like this:

I cut a hole in the back of the bottom stair (as it is in the basement - so can live with it, and so can see how it is attached / supported between the riser and the tread - which is really not at all?

Bottom of riser:

If we then look from the front of the stair (after removing the plywood and the partially attached support on the riser), there are huge gaps up and down the whole stair case:

The riser is definitely in a small slot in the stringer - as is the riser (it seems at least - very hard to tell). The tread has no support at all in the centre, and the widest tread at the very corner is about 1.6m wide. The treads are 35mm thick - not sure what wood to be honest, but I think its pine by how soft it is.

So I would really like to get these less creaky so we can carpet them. I wonder if the wisdom of the community can help with a path, I have the following options in my head:

1. Add a supporting bar under tread at front and screw/glue between tread and riser (I tried this - it didn't seem to do anything, but it may be I didn't glue it well as it was a bit too easy to then remove).

2. Use pocket whole screws to screw the back of the tread to the riser (the riser is behind the tread, not on it). Not sure if this will work as I dont think the riser goes far down behind the tread to get a solid grip on it.

3. Cut out the tread (from above), glue/screw support to the left and right and re-place a new tread. I would also replace the riser though in this scenario I would place the riser on top of the tread (starting at the bottom more like a 'normal' staircase).

4. Cut the plaster off the back of the stair and access the back to glue/screw - this will be really bad due to fact it is curved, plus current plaster is straw/lathe so incredibly messy. Not sure if it will then be worth it.

... any other suggestions. 3 feels like a nuclear option, but is more and more feeling like my only choice. 4 I think for me isn't an option, in that scenario I would just save my money and ask a professional to re-do it all when we are on holiday.

Any help greatly appreciated!

Edit after cutting back of stairs:

Ok, so I now cut more out of the back of the bottom stairs so I can see the actual riser / tread and how they attach to the stringer.

It seems they may be floating a bit more than I expected! There is a notch in the stringer, though the tread doesn't seem to be in it (though the riser is).

I can actually get a knife in the gap between the tread and the notch :| The treads also look quite broken from underneath - perhaps the primary reason for the noise?

The other side appears to be concreted in (to the right is a brick wall). I think i need to look further up the staircase to see if it continues.

Answer 1

From comments, as it appears the staircase was constructed from underneath, then that's where you have to be to fix it.

The slots were likely a retrofit to enable carpet to be run in one piece, allowing takeup of the slack needed to turn corners, in the days before gripper rod, & to make the turn over each tread to look neat, rather than bag slightly if it was held only by carpet rods in each internal corner.

Basically, the entire structure is supported directly on the stringers, each tread fits neatly into a slot/rebate, from behind. This will be shimmed with wedges & cross-braced to prevent flexing. Quite a work of art, a good Victorian staircase. You might be surprised at how few screws/nails were initially used to hold it together.

The risers will need the slots filling, but you need to ascertain how they were originally fixed. They may have been fixed to the tread above using blocks [so no screws are ever visible in the tread], but not below, or they may be attached to both, the bottom tread screwed through from the rear. You won't know until you get in there.
Depending on construction, it may be simpler to take the risers out one at a time to fix the slots, rather than try to jam long wedges in with them in situ. The risers will possibly need to come out forwards rather than rearwards, as the cross-bracing will be in the way - but they are very unlikely to be rebated like the treads, they'll be flat-fit.

The intention is to solidify the tread, then let them carry the risers in the same way as the original design. Often, floating the bottom of the riser prevents rather than creates creaks. The stairs are then several independent 90° boxes rather than one coherent monoblock unit.

This is a messy job, no doubt, but you cannot fix it from above, unless you re-design the entire staircase; you've nothing to rest on & literally all your fixings are done from behind originally; that's how they could keep all the faces so clean, before the days of ubiquitous carpet.