He might have bought an early version of the LT-40 kit? Mine (many years ago now) had the (unmodified) wing saddle cut-out at too high an incidence. Factory error. Everything including thrust lines were as per plan. Even the engine was as depicted per plan. Despite the significant amount of additional lead noseweight required to balance the characteristically tail heavy build of the LT with a similar lighweight powerplant up front, the consequence was that with the OS 40FP at full power, it'd climb abnormally requiring abnormal down elevator trim and the associated problems with decalage amongst other things. Your primary problem sounds very similar. I corrected mine by shimming the trailing edge up about 1/8" to correct the incidence and decalage issue. If that is your problem, that will stop the climbing and any associated pitching instability issues.

As to the landings. The LT-40 is an acknowledged floater, but - propping that engine airframe combo right is the key. It's a high drag (slow) airframe with a (relatively) low power engine - which is though, just fine and arguably the ideal combo for the training task. Too high a pitch prop will give you poor airframe acceleration, slower engine accleration and importantly poorer braking at flight idle. After much experimentation I found a Master Airscrew 11x5 suited the engine/airframe combo well. The blade is slim and light allowing good mechanical and aerodynamic acceleration, and the pitch/diameter combo is right for this airframe engine RPM range. A 10x6 won't work well. You might try a wider bladed 10.5x5 Bolly Sportsman which I think would work well. They weren't available when I had mine running an MA 11x5, but I think that particular Bolly would probably work well for your purpose.

You can research the aerody of it all at your leisure, but you'll find those two suggsetions will probably fix the climb rate/angle at full power and address the descent rate at flight idle issue as well offering good aerodynamic braking for a slow good angle approach with superior engine and airframe acceleration should a missed approach be necessitated. PS: Just read your mention of downwind approachs. This is of course an aviation no-no both IRL and R/C. Expect a floater to extend on approach & float much further on a downwind approach and landing. This exacerbates the importance of correcting the salient factors previously mentioned

Hope this assists.