Lime Mortar / Pointing query

[ray.birch]

Hi all - and firstly apologies if this is in the wrong place but hopefully addresses my question!

We've recently had a loft conversion done, and as part of this, have put aside a section of the eaves for storage.

The builders have plasterboarded out most of the eaves area, but have left the brick walls (party walls on both sides) exposed.
Ordinarily I wouldn't have an issue, but the mortar is in poor shape, and becomes very crumbly and dusty when touched.

A few questions:
1) Does the original mortar look like lime mortar (from what I've read, this is likely in a 1910-1920s house)?
2) It seems like the bricklayers when putting the new RSJ in have repointed in small sections - but seems as though regular mortar has been used - is this likely to be a problem in these small sections?
3) How best to "seal" the wall against more dust and dirt? Originally I'd thought about using a stabilising solution and painting over, but from having read more, believe that the walls may need to breathe? If this is the case, would a lime mortar render (and perhaps a whitewash after) be the best bet?

Apologies for the lengthy post - any advice very gratefully received!

 

[bobasd]

forget about any pointing unless its allowing air in - in that case use sand & cement or sand & lime to do a little pointing. its no big deal.

you could cut plaster board to fit, and then use any kind or adhesive to blob the back of the board, and press the board into position - just like D&D.

fwiw: it seems like a chimney breast was removed from that wall but the soot covered flue bricks were not cleaned off with a wire brush?