I think--Mark might mean pull 'that cover' to hold the lever behind to crack the nut free if it's just won't budge.
By internal adjustment i think he means 'that' pushrod adjustment.
Applologies for jumping in & being presumptious Mark, [is that a word] & happy to be told off.
Just trying to keep it moving for OP incase Mark has the day off.

I don't think I said "front"? RT325 is correct... the cover I'm talking about is still on the bike in your picture and the internal adjustment is the locknut and screw in about the center of the cover below where the cable enters.

If I tackle removing the remaining cover today, I need some help with how to do so. I haven't been able to find much online. Attached is another photo.

drive.google.com/file/d/1gVjHggM9QFYHbqq...W03/view?usp=sharing

1. Remove the red screws?
2. Remove the green screws?
3. Remove the gear shift lever?
4. Remove the clutch cable (purple)?  If so...how?

Remove blue,then red and cover will come away. There will be an arm on the inside that the clutch cable attaches to. Unhook the cable and you can then move the cover to the bench.  You will probably need to unhook cable from the handlebar clutch lever first before trying to unhook it from the arm.

Hold the arm immobile and you should then be able to crack off the locknut.

Without looking at my AT1 which has a similar earlier design--i think you might find a tin cover shielding the arm & cable behind that big cover. If that's the case you'll need to remove 'that' to access the arm & then just limit the arm upward movement with something to then crack the little nut free. There'll be a short small spring oin the arm--if still there--so hook it off the arm. They get stretched & useless anyway.
I've never had trouble just forcing the nut till it give way, which i guess isn't good if it's coil binding the pressure plate springs but i've got away with it.
Little nut should free easy, & don't overtighten it.

Took off the cover and was able to get the nut free...finally! Adjusted the set screw per the manual. Daughter took for a short ride this evening with the same result. In first, she hits the throttle and the RPMs go (6000ish), but the bike won't ...maybe gets up to 5 mph. I thought she was in a higher gear the other night when this was happening, but she said she was only in first. 

Spark plug is new. Carb has been cleaned...twice now. I made slight adjustments to the throttle cables. Could it be the clutch plates?  If so, what are next steps?

Once again, thanks for all the help.

On the face of it if it's as bad as you say it sounds like the clutch is burnt out--fibers slipping that bad that they've burnt the grippy bits off. Alternatively if someone has been in there & assembled it wrong then maybe, but hard to imagine how it could be "So" wrong even stacking the plates wrong so long as they're all in there it'd grip better than it is now, in my warped opinion.
Sorry to say--ya gonna have to go in--unless i've missed something obvious.
Just a thought--have a wiff of the oil by pulling the dipstick. Burnt plates smell yuk. If you've already changed the oil did that stand out as smelling worse than me burning toast in the morning.
Right side cover of course if you 'do' go in. Hope someone comes up with a better plan but i can't think of one.

When I changed the oil last weekend, I didn’t think to smell it unfortunately. Just weird that it seemed to run pretty good for a bit, and then it didn’t. Is this a sign of a bad clutch?

Should we have an experienced rider take it for a spin? Could this cause further damage? Just not sure how to tackle this.



 

Is it worse ( slipping) more since you adjusted the clutch  

Park it against the wall & put it in second then few revs & let the clutch out.
See if it wheel spins for a second or stalls--or just sits there revving.
If it sits there slipping--back wheel not turning--then stop it & report back.
Or good plan you said--get an experienced rider to give it a spin down the road etc.