Noidea about leather...
Cleaning the lens:
Alcohol is safe to use, but not especially good. It isn't volatile enough, so sometimes leaves drying marks.
Best to use a wooden cotton bud / Q-tip. Use a bag of pure cotton wool and pull little tufts off with the unused end of the stick (forget the end with the ready made bud on it) and roll them into a bud against the inside of the cotton wool bag. This way use can make dozens of buds, use once and throw away. You'll need a pure solvent. Methylated spirits are useless as they leave residue. NEVER use Acetone, it seeps under the lens mount and takes off the blackpaint as well. Propanol or ethanol might be the best that you can get. Reagent grade petroleum Napther / Petroleum Benzine cut with about 10% ethanol is perfect, but not easy to obtain. Napther / Benzine (same thing, depends where you live what you call it)is basically lighter fuel, but the stuff in cans for cigarette lighters is far too oily, you need to get pure reagent grade from a chemical supplier or pharmacist. The gold standard was Diethyl Ether, cut with 10% ethanol. All reagent grade and water free - but dangerous stuff to play with (very volatile and inflammable and obviously an anasthetic). With modern safety regulations and so forth it is pretty near impossible to get, unless you happen to work in an industry that uses it?
Moisten the bud with the solvent of your choice, start in the centre and go around in a spiral to the edge, replace the bud, repeat as many times as you need to... Be careful to 'wash' the lens with plenty of solvent. Use a dryer bud when it is nearly clean to get less drying marks. Never rub a lens with a dry cloth, it will scratch it. If you have really stubborn gunk on the glass, soap and warm water under the tap with a cotton bud to loosen the stubborn bits shouldn't do any harm to unscrewed elements, just use common sense about getting water where it shouldn't go.
You'll need to reset the distance scale on the from element when you refit it. Use a bit of ground glass and focus on a bright light at (say) 20ft or something and then slacken off the three screws securing the outer scale and set it to 20ft. Then check infinity...
HTH