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We use batten and counter battens on tile roofs always. Same difference for slate. Fir is probably best, don’t use cedar over a counter battens it does not have enough strength, but it is more moisture resistant if placing it direct on the deck. Pressure treated is OK but, in my mind, not mandatory as once battens are elevated, they don’t pond water even with broken slates above and are protected from UVs by the slate with air movement around them. As far as strength, if using counter battens the closer you put counter battens together the less deflection on the batten and the greater strength. I really like the Boral (now Westlake) Elevated ax3 battens as they have supports every 12 inches so almost no deflection and no bonce when nailing.
Here is a photo of job we are doing on Mt Hood it has PT 2×4 counter battens 16” on center vertical, a draped underlayment over those and Pt 2×4 horizontal battens with inch thick slate. This sometimes gets 12 feet of snow. It also has highly permeable Vapro Shield self-adhered membrane under the counter battens. We got snowed out so going back in April or May when it melts.
Also here is a batten span chart from the 2002 Tile Roofing Institute Moderate Climate Manual which I helped with the writing.
Looks like efflorescence to me. That is salts and lime milking out of the mortar in the chimney. Most likely originating from the mortar cap on top of the chimney if it is exposed. A 10 to 1 mix of muriatic acid and water is what we use on concrete tiles for removal of efflorescence, but not sure what that would do to the copper flashing. Might be Ok if rinsed off after application. Might also take a stronger mix.
Suggest placing a copper cap over the top of the chimney and sealing the sides with a liquid applied sealant. Maybe tuck pointing as well? Suggest talking to a good masonry restoration company.