Like Tom, I do not have an engineering-type answer. I do have my horizontal lumber stored on shelf bracket/standards similar to these. There are three screws per vertical and I have hundreds of pounds distributed along 16 feet of wall.
I also have hundreds of pounds of tools and clamps mung from french cleats with 3/4" hardwood ply cleats as you describe.
Hopefully a licensed builder or engineer can chime in with some raw math.
I'd use 3 coats of Arm R Seal or Minwax Wipe On Poly and be done with it. If the handle is too tacky, rub it with 0000 steel wool after about 2 weeks.
As suggested, Danish Oil might be a good choice, I have never used it so I can't recommend.
As a word of advice, use 3" lag screws, and make sure you are in the center of the studs. I've hung some pretty heavy TVs using steel TV mounting brackets. I have a 100 pound TV hanging on my wall using 4 screws, 2 in each stud through the mounting plate.
Another reference to heavy things on studs. My lumber rack is fastened to 2 different studs with 3" spax screws and only 3 per vertical support. I've got over 300 pounds on my lumber rack easily.
I am building a new bookcase that will literally just be a series of boxes, around 14" tall, and 48" wide, and mounted to the wall using french cleats. I will be using 3/4" thick birch plywood veneered in cherry. The French clear will be also 3/4" hardwood plywood, and will be long enough to be screwed into at least two studs, maybe/hopefully 3 studs. I have never tried to snap or deform a 2x4 stud, and I certainly do not want to try now. Is it possible that a bookcase of this nature will cause any damage to the studs? Oh, and I plan to make another one just like it, and hang it 8 inches or so above it, and about 12"-14" to the left. Very Mid-Century Modern design, huh?
If studs are placed every 16", and my box-shelves are 48", I should be able to hit three studs with each box. So, there will be 4 studs involved, with the middle two studs having both boxes anchored to them.
I do not own my home (rent), so I am sure you guys can understand my hesitance. Do you guys think I'll be totally fine? A 48" wide bookcase loaded down with thick heavy hardback reference books will weigh a lot! Trust me, I've have to move the boxes three times in one year. I know how heavy they are.