I think this is largely correct. You would not even need to plug every gap to bring these combos in line, presumably just tighten up the wording in the multiclass section. So specifically, if we are going to target just these combos, what additional sentence could be added to the multiclass section to bring these combos into line?
Sorlock: Warlock spell slots may not be converted into spell points. Sorcerer spells may not be converted into Warlock Smite damage.
Hexadin: Warlock spells may not be converted into Paladin Smite damage.
Sorcadin: Paladin spell slots may not be converted into spell points. Sorcerer spells may not be converted into Paladin Smite damage.
Looks like Eldritch Blast is becoming a class feature, which might fix some issues there regarding multiple beams and level dips/feats.
Anything else?
NB: This is to reign in overpowered combos. I think Smite damage needs to be fixed more generally. It doesn't preclude feats that enhance multiclassing in some way.
I have never seen a problem with any of those things in play. I have seen a lot of theories on how bad they are, but we have seen those class combos in play and never had a problem. Part of the reason why is smites are generally a weak use of a spell slot compared to what you can get out of an actual spell of that level.
I also think there are many single class Wizard builds and a few Cleric builds that are quite a bit more powerful than those "OP" combos.
Multiclassing Sorlock for sorcerery points is a particularly bad idea. You would actually have more Sorcery points if you just were a single-class Sorcerer and used the higher level slots you don't have with the multiclass for sorcery points. For example using a 2-short rest day on an 8th level character:
Warlock Dip-Level 6 Sorc/Level 2 Warlock: Your Warlock slots will give you 6 SPs per day, but if you were a single class Sorc you would have 2 more to start with and 2 4th level slots that could give you 8 more on top of that.
Sorcerer Dip - Level 3 Sorc/Level 5 Warlock: Your Warlock gives you 18 more Sorcery Points (assuming 2 short rests) assuming you never want to cast a spell higher than 2nd level, but if you were a single class sorcerer you would start with 5 more and your extra spell slots (1-2nd, 3-3rd, 2-4th) are worth 19 Sorcery points. So casting the same spells, the single class character has 6 more SPs.
Equal Multiclass - Level 4 Sorc/Level 4 Warlock: Your Warlock slots can be converted into 12 more SPs, but a single-class Sorc would start with 4 more and have extra sorcerer slots worth 17 more.
In all 3 of these examples you are losing SPs per day with the multiclass and that assumes an ideal 2 short rests a day.
Also. RAW Sorcerer spell slots can not be used for Eldritch Smite. That can only be done with Pact Slots:
"Once per turn when you hit a creature with your pact weapon, you can expend a warlock spell slot to deal an extra 1d8 force damage to the target, plus another 1d8 per level of the spell slot, and you can knock the target prone if it is Huge or smaller."
I really hope they do not make EB a class feature for two reasons. First I don't use it on all my Warlock builds and second, it is nice to have it as an option on things like Magic Initiate.