I think your getting to hung up on the lime situation
If there’s cement and lime in the mortar then the lime is just used as a plasticiser, it’s not doing a deal to the mortar.
Yours just looks like sand and cement to me, how do you know there’s lime in it anyway?
It looks like cavity wall?
Yea I think I am.
The doubt has come from when it was done and some boards failed but I think this was just due to no suction control when it was done.
I am thinking to dab and mechanically fix with insulated plasterboard fixings? Just can’t afford it to not be done properly.
Regarding the lime, it is a cavity wall but if you do a vinegar test it bubbles a lot which indicates lime in the mix
Would you dab or float and set? As long as it lasts how it should I’m fine with either