as jester says, the game isn't about inventory management, so i don't really consider infinite stacking as making the game easier or harder. multi grand mastery (and most components of ease of use) aren't going to make BP harder or easier, either - they weill however break yr install.
as for making BP easier - anything that adds stores, items or spells is going to make BP easier to play. item upgrade, ruad, SP items, magnificent magic shop, exnem's items, FW items, dark ritual,
FR/ROV and the planar sphere all spring to mind (as i have them all installed

) - although at least the planar sphere mod gives you a couple of realy hard battles, to somewhat counteract the benefit of the spells and the items.
the nerfs in the s_and_h patch (although *not* the sureblade nerf, which should really be described as a fix) also make the game easier (or less frustrating, depending upon yr point of view).
the st. cuthbert's mace you picked up after killing the shadow dragon is a basic part of BP, isn't it? or is that molytyr, which you get for killing the unseeing eye? at any rate, BP (or rather, its components) adds a whole heap of new items and spells, which definitely assist in completing the game - check out the weapon you get for completing
NEJ, as an example... not much you can do about these items and spells though, except not use them ,as they're built into the game.
so if you want an even *greater* challenge, don't add any mods which add stores, items or spells. add those npcs that have tricky quests attached (kelsey, tashia, kiara/zaiya, sola, valen, some more i've probably forgotten) and play their quests through; and, because a number of the scripts in BP summon extra opponents depending upon yr difficulty level, turn the difficulty setting above 'core'.