Demon wall becomes rather easy if you have anything like Haste and/or Protect Materia link with All. Makes the fight rather easy. Or level up a bit near a save point in the Temple.
If it is true however, I assume it would take a lot of fruitless digging before you finally ‘hit paydirt’ so to speak and find any items you missed getting the first time.
As for Demon Wall, if you’d taken the time to get Aeris her level 4 Limit Break Great Gospel, it makes that battle a cake walk. The Enemy Skills Big Guard and White Wind make it much easier too if you don’t have Haste and Barrier materia.
Just try to get full limit breaks on everyone before u enter the battle.
Besides, if you want quick leveling or quick materia masters, why not take ur bronco over to the edge of mideel, those little catepillar scythe things get u 400ap or something per kill and the sand worms are good for 5000gil.
(I have good knowledge of FF7, but havent played in 4years (stupid ps2) )