Is it okay to "cement" around the overlap of slate tiles

on old roofs before modern membranes were invented every batten would would be pointed up with lime mortar after the slates were attached to the battens, it was called torching this achieved two things, it secured your roof tiles into place, and it acted as your secondary protecting like today's membranes catching any seepage of water getting under your roof tiles entering the roof trusses preventing rot & dampness in the roof structure as all tiles leak to a small extent. the reason your slates blew of in the wind is because your roof will have nail fatigue ,basically meaning after this length of time the nails will be weak & most probably rusty this can allow wind to get under them & lift the weakened & unsecured tiles of the roof if blowing & hitting the roof in the right direction there is no way of replicating this as mother nature & her weird & wild forces is something we are still unable to replicate . do not cement these areas as cement will have a negative effect on the roof hope this has help you understand a bit more about your building. good luck Alex

Yes Alex is right it is not a good idea to put more back pointing on from the in side as it will not stop the loose slate from slipping as the nails are going to be rusted as a result of the back pointing