You're correct in that Popopurin is pretty much a better Kempel and Emil, but they all have their uses. The point of elemental conversion is that your elements go up when *you* take damage. Meaning if an Idola goes multi-target happy, you can pretty much bury it with Elemental blasts in very short order.

Also Multi-target attacks from you aren't good if you're looking to zero in on specific parts. If you're looking to do tons of damage/rank higher in Idola Battle ranking, Multi-target is the way to go. Random Target multi-hit is great as well, but you don't see that in Elemental Blast form.

As for limit breaking, it's about a 35-40% difference between 0 and +4. Obviously a 5* is more likely to see 40%, but a Base 4* Event character like Melissa gets 30% from going to +4. I'd say both Limit Breaking and Starting element is pretty important in Idola battles since you are locked to 8 turns worth of action, and depending on what you are fighting, you need to be able to output damage quickly, especially against Idolas that have Elemental Resonance (a skill that makes them gain 4 points for all elements). The faster you can output damage, the better off you are, especially since 1 turn can spell the difference between exposing a core and surviving, or taking a bad hit and watching it all snowball to a party wipe. At the same time the higher stats will help your survivability, though I dare say in the face of an S rank Idola, the Starting element value will help you far more than the stats, since one Elemental blast from most Idolas will most likely kill one or more characters, Leo and Aries especially. So the faster you can muster the firepower to deliver damage via access to better attacks and buffs, and maybe throw in an elemental blast, the faster you can expose its core, and prevent it from doing anything. You can use that time to keep attacking it, since you'll do extra damage, or use that time to buff yourself up, and deliver another devastating followup (if that option is available to you).

I'm going to add a third category of importance - Status effects. Idolas can be affected by them, and there are a lot of them that will cause much grief for your enemies. Any characters that can cause elemental sever (icon is a grey hexagon) are a pretty huge asset to fighting Idolas if you aren't confident in your defense - this state causes those inflicted to gain 0 element from their attacks, which basically saves you from having to eat stronger attacks (assuming they don't have the necessary element to begin with) Burn and poison are self explanatory, and with Idolas you always get 3 times the effects, since it will affect every part (also assuming it has more than 0HP). And shock, prevents targets from using EBs. I'd say anyone that has the ability to inflict these statuses are well worth it regardless of limit-breaking. Freeze would be great too... if it didn't take one or 2 attacks to break them out of the status.

tl;dr version: Limit Breaking is important, but in the face of missing limitbreaks for base 5* characters, a maxed out 4* will definitely suffice - and some 3*s as well, since many of them carry useful status effects and buffs (Like Sasha, whose Chaos form has a blast that makes her take ALL of the damage directed at all your characters for a turn, effectively preventing a party wipe), but most importantly, it gets your starting elements up, and it allows you to summon more powerful idolas sooner (there's a reason why when the game first started, the only S rank Idolas all came from 3 and 4* characters as the lead).