Fair enough, sorry this didn't work out! I mean one houserule changes right off the bat would be to make it any spell that flat out does not work, so if it misses or if all targeted creatures make their save, it activates. Another is that Arcanum spells are included, of course. And you could always make it automatic. I hope the game you're in isn't over yet, so you could switch it up and tell me how it goes!

Okay, I've now played this homebrew option to Warlock Level 11. I took a one-level dip in Sorcerer, so I've had "Fallback Slots" for six levels – and I've never once been able to use it successfully.

Fallback Slots is incredibly under-powered. There is almost never a good reason for the feature's prerequisite condition (using a single-target spell) to occur, especially after class level 11 (more on this in a moment). If the prerequisite condition does occur, there is only a 25% the spell slot will be recovered.

First, let's look at the level 6 Patron feature for the official Patrons:

• The Archfey can use their reaction to turn invisible, then teleport away. It recharges on either a short or long rest.
• The Celestial adds their CHA mod to any radiant or fire damage roll, and they gain resistance to Radiant. Doesn't need to recharge.
• The Hexblade gets a scalable Specter minion which lasts until the end of your next long rest.
• The Undying gets automatic self-healing when either succeeding a death save or stabilizing a creature with Spare the Dying. Recharges on a long rest, but automatically healing oneself on a death save is uber.
• The Fiend gets a large bonus die added to an ability check or saving throw, triggered after seeing the initial roll (but before effects occur). Recharges on short or long rest.

So, why doesn't 'Fallback Slots' work well?

• There are few single-target spells in the Warlock spell list – even considering Expanded Spell List options.
• Even if you are willing to give up one of the Warlock's precious spell slots to cast a single-target spell, you only have a 25% chance to successfully save the slot. By that time in a session, given that Warlocks only get 2 slots before class level 11, you're probably out of slots.
• This means you'd have to use single-target spells habitually in order to see any mechanical benefit from the feature. You'd have to use four spell slots on single-target spells, on average, before seeing one spell slot recovered.
• This also rules out the idea that this feature is primarily meant to be used single-target out-of-combat spells – especially as most of those, as well as the single-target crowd-control or debuff spells – become multi-target when upcast – which Warlock spells automatically do.
• Among the Warlock's single-target damage spells, some of them don't squarely beat Eldritch Blast for damage, even without the buffing Invocations. Only two beat EB for damage with Agonizing Blast, except in the very most niche of circumstances where you are trying to get a secondary effect to trigger. None of them beat EB for damage, when buffed by Agonizing Blast and Hex. And if you have the other EB-buffing invocations, then your EB is doing more than just damage anyway – it's doing automatic secondary effects.
• This gets increasingly worse the higher level we go beyond class level 11, because Warlocks never gain spell slots beyond Slot Level 5, meaning that damage scaling for spell slots for Warlocks stops now, while damage scaling for EB and other cantrips continue.

I like it, but I think True Namer is a bit confusing and maybe under-powered. I also think the DC for Fallback Slots should decrease as you go up in levels, or benefit from your Charisma modifier, or something like that.