Lots of questions, and hopefully I can help you piece things together. When combining weight training with any kind of a sport, you do have to keep in mind recovery. To that end, you have a couple options:
- Tack it at the end of the week. This is a pretty sound idea. You still have a day of recovery between then and the next session. I used this approach with incorporating martial arts. This works very well when the activity you want to do is also high intensity.
- Tack it at the end of weight training. This also works while the weights are relatively low (if squats are below body weight that qualifies as relatively low). When your squats get closer to 1.5x body weight, the two training sessions will affect each other. I also tried this when scheduling forced me to.
You can use both of these approaches depending on your schedule. I do not recommend trying to sandwich rock climbing on the rest day between weight training sessions.
is there anything I should do in the intervening two weeks to help me with my rock climbing later?
Two things that help with rock climbing are core strength and grip. Two of the biggest things gained with deadlifts are core strength and grip. They are a match made in heaven. In fact, getting better at one will help you get better at the other.
I understand your concern with deadlifts, I used to have a bad back myself. The key to deadlifts is to keep your lower lumbar region rigid. Do not round your back at all. Also, it is worth trying both sumo and conventional deadlifts. You'll find yourself feeling stronger with one or the other. Whichever one is stronger, use that every time you deadlift. Whichever you choose, the principles are the same:
- Setup: bar over mid foot (about 1" in front of the shin). Grab the bar, straighten your back, and pull the slack out. NOTE: you should raise your shoulders to pull the slack out, not your hips.
- Brace: tighten your core and your grip, shift your weight on your heels, and start the Valsalva maneuver.
- Pull: your back angle should remain the same throughout the lift. Push your heels through the floor, and activate your glutes and hamstrings to help you stand erect.
- Lockout: tighten the muscles between the shoulder blades, and body should be fully erect. Do not overcompensate and push your hips forward giving you a backwards lean.
- Drop: it's best to have a controlled drop rather than trying to set the weights down like they are made of glass. The deadlift is all about the concentric phase (the pull), and really does not have an eccentric component.
One option to help your grip even more is on the last rep, hold the lockout for as long as you can. You can breath during this time, and in fact this is one of the better ways to teach yourself to breath while exerting yourself.
NOTE: during the entire lift, the only job your back muscles have is to keep your back straight. You change the angle of your back with your glutes and hamstrings. This will protect your back from injury. I'm pulling over 405lb on deadlifts now. Occasionally I have a back pump, which while uncomfortable is not the same as a back injury.
Option 2: Farmers Walks
Farmers walks have a heavy weight in each hand, and you carry that weight over distance. Your body needs to be erect, and try to prevent your shoulders from dropping forward. These do well for conditioning as well as building your grip strength. While they do improve your core, it is not to the same degree as the deadlift.