Maintenance I need suggestions for maintenance for Sujan

For the head magnet I know for sure that superglue works fine. It holds up. Just be careful with the right polarity!

 

Craft glue normally won't hold magnets even in low humidity so seconding you'll need to use super glue. Just make sure you let it dry well before you put the headback and faceplate together so the magnets stay put in their slots well. If the elastic tension holds it in place well, you probably won't even need to glue down the rod in the headback (but maybe some Limhwa owners can chime in on that).

Elastic degrades over time and has been under tension for 10 years even if the doll hasn't been played with - it's not humidity that made it behave differently, it's just old and becomes floppy fast no matter how you tighten it. You'll need to restring with fresh elastics and it should be all good.

Since it is a professional faceup it has definitely been sealed so there is no need to worry about that! You can fix the bald spot if you want (I'd just seal the chip with a drop of brush-on sealant, fill in the spot with acrylic and seal/gloss again) but if it doesn't bother you, you can also leave it as it is or add some gloss or sealant on top just in case.

 

I'm not sure what sculpey you refer to, I can't see any on the photos. The stuff holding the eyes looks like some kind of silicone product. If it's soft it may be ear plugs, if hard it could be hot glue. Either of those should be fine to leave in unless you have silicone eyes in the doll. But I'm not sure if that's what you are talking about.
The lashes looks fine on the photos.

I second super glue for the magnet.
The rod should not be glued in. If it is you can't remove the head without taking the entire doll apart and that negates the point of the forward facing opening in the neck hole. Working with a rod instead of an S-hook is a little more difficult when removing the head, so if you are going to do that often (when dressing the doll, or for head swapping) you may want to replace it with a hook. If an S-hook doesn't fit in the head cavity, a C-hook or even a fixed ring can work as well.

 

You already received great advice on your inquiries.

My two cents as a lucky owner of a Limwha Limho Mano; I removed the glue holding his bar, as soon as I was able to. Having a faceplate system makes Limhwa's dolls a dream (for me) to customize, but my collection of floating heads made a glued down bar a nightmare.

I never had any issues removing the bar, nor it staying in place after ungluing it. The faceplate makes it so that the head's back piece is not an issue for dressing, undressing, switching eyes or customizing. It just makes it easier, if you don't glue the bar, if you need further customizing (like body blushing, restringing, sanding, etc.). Or just have other heads that might possibly share the body.

Otherwise, I would say don't glue the bar, it doesn't go anywhere, when properly strung and you don't need an "S" hook, as an alternative. As I mentioned, I own a 67cm Mano (not to be confused with the tinier adorable Mono!). So I can't say for certain how well the girls stand, but my Mano stands like a rock, when his elastic is in good condition.

Congratulationson your Limhwa doll, she is beautiful! I have a soft spot for all Limhwa/EOD BJD. I wish I owned more! (:

 

Normally, 1/4 dolls are strung with 3,5mm string in the body and 3mm string in the arms so you're right about them being different.

The channels may be slightly different in size due to the hand-made nature of dolls, if the crochet hook doesn't go through, I'd try pulling the elastic with ribbon or a piece of wire instead.