The original ICV-K1 yeast you used was a good choice.
I would not switch to EC-1118. There is a possibility, however slight as the K1 colony has stalled, that any remaining K1 may end up interfering to some degree with the EC-1118 ... K1 yeast is known as a "killer strain" for it's high competitive factor. No sense in doing something that may be less desirable in practice.
Your list of what you've done suggests you did things correctly ... HOWEVER you stated that you used "lemonade concentrate" ... a sweetened product ... and sugar for a final volume of 1.5 gallons ... my calculation for your recipe shows an Opening Gravity of at or above SG 1.140 and a potential final ABV of over 18%.
Too high a gravity causes a pressure imbalance for the yeast that eventually causes much of the yeast colony to die. The K1 you used is pretty good at chewing through a high sugar/high gravity concentration but generally requires stepped nutrient additions and stepped sugar additions to do so - and especially in a lemon wine.
With the low pH of the lemon product ... and then racking the must away from part of it’s active yeast, it was enough to stall it.
If you don’t like the potency (ABV%) and flavor at SG 1.05 and want to resume the ferment, make a starter with more k1-v1116 (the yeast type you already used), get the starter rockin & rollin ... add an additional 1/6 (one sixth ... basically just a dash) dose of nutrients (but no more than that) to the must ... then aerate your must by vigorously stirring or shaking to re-oxygenate it, and add the active starter. Keep at a mid-range temperature. This should finish out your fermentation.
