Firstly the dance studio, car repairs place, furniture depot and RP of adjoining block have main responsibility as employers of persons by virtue of tenancy or similar having control.
All should do FRAs, which should consider other relevant persons (i.e. each other potentially) and co-operate and coordinate with each other regarding fire safety.
The landlord may have no control if there are no internal common areas and thus not need to be interested - whose is the stair - just because it is shared doesn't mean it isn't under the control of one of the tenants by virtue of the lease, if it's the only 'common' thing on the whole site some landlord's will demise it in order to lose any day to day responsibility.
Shared escapes between totally different buildings brings the spectre of easements (or lack of) into play as well.
Without visiting the site and knowing the extent of the leases it's difficult to be exact, but I've been to a site with multiple tenants above and alongside each other where the landlord controlled nothing (all demised) & just collected rent, the IO was happy with an FRA that pointed that out and included tenant advice regarding their own FRA and need to cooperate.
