If the concept of this is a curse/punishment to dragons to lock them into humanoid form, why are they gaining dragon, half-dragon, and dragonborn feats?
If this is to be some kind of "class feature", then make it a feature (taking up a slot/removing something else) and limit it. (I'll try to incorporate it below).
First, 5e Classes begin with 2 1st level features. Given the nature of this character archetype, one of them must be a choice of what kind of dragon you are and what form of humanoid you appear. This choice, by definition, is going to present starting features.
Second, the comments on the bumped-up "unarmored defense" [which is already a defined feature in 5e] have already been called out. ...and with a shield (uh, is that how unarmored defense works?)...and an extra HP per level (why?)...
Third, what is this "half blindsight" business. They've been cursed into a mortal humanoid form...They get whatever senses/special vision that form has. You want to augment it somehow, fine. Make some kind of Dragon Senses bonus? I can see that. A bonus to Perception rolls. Or even Lowlight vision even if they appear human. But a 30' Blindsight?! What mortal humanoid species gets anything even close to that?
Fourth, if you are in mortal humanoid form...why would you have Natural Attacks (to begin with anyway) beyond just an unarmed strike, like any human, elf, gnome, dragonborn, etc... Forget about the extra energy (of two types?) damage from them...that, oh yeah, also applies to your weapon attacks? What?!
If you want to give them "natural" attacks (like a claw attack) that deals extra damage. Do that. If you want their unarmed strikes to have energy damage, do that. If you want their "natural/unarmed attacks" to count as magic for the sake of bypassing restrictions and immunities, do that (Monks can do that. No big...at 6th level). If you want them giving extra energy damage to their weapon attacks, do that.
But those are ALL separate features that can grow and develop with the character. You can [should] not triple or quadruple up all of this extra power in a single 1st level feature and not expect calls of "OPed shenanigans."
Fifth (we're still in the 1st level features, aren't we?), per the functioning of 5e, if you have Warlock progression, then use the Warlock spell list. If you are giving them Sorcery points and Font of Magic, then use the Sorcerer spell list (with energy type restrictions is all well and good). But using X progression with Y spell list is sloppy (albeit simple, I'll admit) class design. Make a specific spell list for this Dragon Locked in Humanoid's Clothing Class...and while we're at it, how about a simple name for the class that's a bit more evocative than "Humanoid Cursed Dragon."
If I had to choose...off the top of my head, for a starting point... I'd probably go with 1st level features:
Draconic Origin: Once chosen, can not be changed. Choose the kind of dragon your originally were (and may yet become).
- Grants (normal) Unarmored Defense.
- Grants Preternatural Strike: Your unarmed strikes count as magic attacks for the purposes of bypassing resistances and immunities.
- Grants Preternatural Resistance: Based on the dragon type of your origin, you gain resistance to the energy form of that dragon.
Humanoid Prison: Once chosen this form can not be changed.
- You gain any resistances, weaknesses, and the senses of the humanoid form. You do not gain immunities or any other racial traits (beyond appearance) of the species.
- You count as both dragon and the humanoid you are for the purposes of all bonuses or penalties from items, attacks, etc... dependent on either.
Sorcerers gain Font of Magic at 2nd. Ok. Fine. But that's all they get at 2nd. Metamagic doesn't kick in til 3rd. Making a Dragon more magically adept than a Draconic Sorcerer seems like...I don't know...it makes sense, I guess... but also seems entirely unnecessary/redundant.
I don't see how it's "limited" when you are building in a sorcery point recovery mechanic that Sorcerers, themselves, do not have. No reason you can't regain sorcery points through Font of Magic, as the feature is written.
As a general rule, as some others have pointed out (e.g. Extra Attack better or same as fighters), you want this class to be "sorta like" a combination of different other elements. That's totally fine and pretty common for homebrew class-making. But generally, it is considered bad form, if not "overpowered/bad design," to make a homebrew class that is a "better X than X." This additional sorcery point recovery note strikes me as that kind of design faux pas.
Seems like this should be where your Spellcasting/Progression begins.
I don't know enough about existing 5e "spellsword" type classes to know if this is accurate or as written for existing options of this type of attack/feature. My gut says that it makes sense that THIS is where you could/would incorporate the "extra energy damage" based on your dragon origin type. Spend a sorcery point and add the energy damage to an unarmed or weapon attack. Period. No bonus spell casting.
"Draconic Talent"...now we're getting spells like a wizard...that take 4x as long to prepare [?} and expensive... This is just...I dunno...I don't get it. I don't see the appeal or reason it is needed as a feature...or make THIS the mechanic by which you get to do the "spell as a bonus action" thing with a weapon attack.
This is fine. 4th, 8th, 12th, 16th,19th Ability Score bonus (and Proficiency Bonus increase). Standard for all classes. No problem.
Isn't this basically the same feature as the Spellsword (above)? Granted I think that one should be changed, so I guess this one makes sense to leave.
And Extra Attack...once...fine.
This looks ok, I guess. I do not understand the "dragon energy or force" thing. You are a dragon. Your other features are tied to your dragon energy type. Elemental affinity as defined for draconic sorcerers is tied to your dragon energy type. What is justifying the "force damage" option?
My preference would be to alter the initial feature as noted above and so this would become a damage increase...and/or add "force damage" as an option here, too.
You who what now? One spell from each spell list?! No class learns that many new spells in a single level. Make it an option to learn A [1!] spell from A [1!] spell list other than the sorcerer's (or this class' custom list).
Nope. Fighter's shtick. Come up with something else.
For a 10th level "milestone" kinda feature. I'm actually ok with this. But it would just be "Draconic Attack" because I would not/never all the "Dragon Attack I" feature at 1st level.
I"m thinking "No" on this too. The whole use of Metamagic by this class needs a rethink and restructuring.
Is this commensurate with whatever the dragonborn breath weapon does?
I don't mind the Returning Form. I think it works. It's flavorful. It's not OPd for the level...perhaps even a bit late, imo.
Elemental Mastery is ok, too, I guess.
The problem, of course, is that classes -especially spell progressing classes- do not receive multiple features per level.
Huh? At 14th? No. Fighter's thing. Ranger 16th. Sorcerer 16th. Warlock 16th. Paladin 16th. Dragon Cursed in Humanoid Form: 16th.
I don't "hate" this...I think it's...hmmm...It needs some reworking/refinement. I see someone mentioned something about features based off HD not being a 5e thing...and that seems legit, afaik. I'm sure there's a way to make this useful/happen without relying on that element.
Keep Returning Form. Drop (and see above re

Metamagic. Again, 1 feature.
This is just a no...again. This needs redefining.
Yeah...if this is a third increase of a revamped "Dragon Attack" then fine. The Dragon Form makes sense for flavor...though I am confused by the once per day and don't tihnk it's necessary. Make it dependent on spending Sorcery points for every 10 minutes...and you don't gain 4 extra Sorcery points on top of this.
All in all, I dig the idea/concept. I just think that you are trying to stuff too much stuff in too many places and the result is an unnecessarily complicated and blatantly overpowered first pass. Refine. Rework. Reposition certain elements. Prune and crop. There is a decent/cool class here. It just needs work.