The main problem with Planar Turning is that you need +1 rebuking level per point of SR the outsider has. You basically need to pull UMD shenanigans with a phylactery of undead turning to get a high enough rebuking level to command anything worth bothering with. As an example, let's take the balor. Balors have 20 HD and SR 28, which means that you need a rebuking level of 68 in order to successfully command one. The same goes for devils. Pit fiends have 18 HD and SR 32, meaning they also take a rebuking level of 68 to command.
With a rebuking level of 45, you aren't going to be able to command anything bigger than a bone devil or a glabrezu, neither of which are going to matter in epic play except as message carriers or scouts (bone devils are halfway decent at this, with fly, invisibility and greater teleport at will). Sure, glabrezus have wish as a SLA usable 1/month, but if you wanted wish you'd just command a horde of efreeti. Both are equally useless in actual combat but phenomenally useful as utility monsters. They still won't make a difference when it comes time to actually fight a demon lord, archdevil or deity
If planar turning didn't have the SR-as-Turn Resistance clause, it'd be disgusting. As is, it's mostly unusable unless you want minor outsiders doing your household chores and serving as messenger-boys (greater teleport at will is useful for something).
Since the OP is going RKV, Divine Defiance is about the best damn feat you can take behind DMM. Make an epic version of dispel magic (preferably with scaling CL but anything with a high enough cap will do). Make it take a retarded amount of time to cast, or some other crazy mitigating factor. It doesn't matter because you're never going to cast it. What you're going to do is prepare one copy of it every day and use Divine Defiance to counter people as though you were casting your uber-dispel. Since immediate actions take up your swift action if it's available, RKV can give you as many counters as you need per round. Grab Mordenkainen's disjunction as well, just to have a single copy prepared to auto-counter it, just in case.
Also, take a look at the Lightbringer feat from Races of Faerun. You have to be a human from Damara (though your DM seems like he'll waive the regional requirement) to take it, be able to turn undead, and be able to cast divine spells. The feat allows you to spend a full-round action and 3 turn attempts prior to casting a spell to increase that spell's CL and DC by 2. You can take as many of these full round actions as you want, but you have to cast the spell the round after you finish charging. Assuming your DM will allow nightstick stacking, buy a pile of them (like, say, 600+) and spend an hour or so charging. That will net you a +120 to CL and DC. Go Shadowcraft Mage, mimic an arcane genesis and make your own plane where 1 hour there = 1 round normal time. Pop into your plane and spend an hour charging, then drop a quickened gate before you cast your big spell. Toss your pumped-up spell through (holy word and it's ilk work well, probably heightened to get around greater spell immunity) and assassinate your target with no save (or an insanely high one if they have Divine Denial). For extra cheese, grab a wizard cohort who was born on your genesis plane and have him cast planar bubble. Now you can pull this trick off from wherever, with essentially 1 round (real-time) of preparation.
It's actually very easy to build all of this into a gestalt character, esp. with your DM's loosened restrictions on gestalt. You can get 20th-level spellcasting, ScM 5, and 20th-level IL all in one build with levels to spare and grab all the above. If you can, try to grab some Sovereign Speaker levels to snag domains that grant different types of turning. Lightbringer requires that you expend turn attempts, but fails to mention a particular flavor. As such, if you can grab the various elemental domains alone, that'll net you 4 extra pools from which to charge your spells. Alternatively, get a nightmare (bind it, rebuke it if you go that route, whatever) and have it use astral projection on you. This lets you manifest yourself to the various planes to get fresh copies of all your equipment, including your nightsticks. That way, you can persist all the buffs you want, then refresh your turn attempts and go adventuring knowing that you can counter people virtually at will or toss off supercharged spells at a moment's notice if needed.