Move the post to arsenal. :)

"Antiqua or Sealed Sword" is the wrong question. You should always think in terms of 5-star items, because that's where everything ends up (unless you're focused on Tier-2 Lockdown), and you don't want to get stuck in a lame alchemy path (e.g. Winmillion).

So a better question is, "Divine Avenger, Gran Faust, Argent Peacemaker, or Sentenza"? The answer depends on whether you want elemental or shadow damage, and whether you want a sword or gun. Your plans so far are really vague. You plan to get a Nightblade (Acheron) or another Brandish, but the other Brandishes are elemental, so Nightblade is not comparable to them. You might find my detailed sword guide helpful as you think more.

Another better question is, "Do I want any Jelly Gem items?" Maybe you'd be better spending 15 Jelly Gems to get a Brute Jelly Helm, and selling that to a vendor for 3,500 crowns.

sell it and get the 3.5k cr.

sealed swords are outplayed by brandishs. sealds without asi UVs will cause you lots of touble because they´re sloooooow.

sentenza is a viable choice for fighting gremlins and argent is a nice wep to get rid of chasing howlitzers.
(the drivers are noticeable options there, too, and they deal more dmg. not a single serious gunslinger i know uses antigua line)

=> get the cr, go to the AH and buy those with ASI UVs.

edit:
if you wanna go on farming JK get faust
if you wanna defeat Seerus get Sentenza
if you wanna clear FSC get Argent Peacemaker
if you wanna solo FSC get Divine Avenger
if you wanna defeat vanaduke get blitz needle

You ask whether it's best to get "temporary gear" to beat the Royal Jelly, or whether it's best to skip toward your final gear. Aren't you already able to beat the Royal Jelly? Then why do you need better gear for it? Are you spending a lot of energy on reviving? If not, then stop thinking about RJ, and focus on your future, even while you continue to farm RJP.

The Sealed Sword lines have their uses (big knockback). The Antigua guns have their place (easier to use than the alchemers, no Polaris knockback).

To reliably beat JK you could do dusker or chroma set, twisted targe, and vile striker -- however the vile striker is a 4* weapon.
I found it harder to solo JK with Faust (4* sealed sword line) than with vile striker - faust only curses on second hit / charged attack, while vile striker pretty much constantly poisons. I can't think of a 3* weapon I'd take to that fight. It's a much easier fight with 4* armor and shield, of course.

You'll prefer chroma set to dusker only if you plan to use guns and bombs a lot on you Jelly Runs. Dusker is actually useful outside of jelly palace with good 5* options later on.

To respond directly to Hawxindanite :
1. and 2. Do "in the long run" means FSC ? If so, yes. But in the long run, you need both, because they have different damage types and you'll need BOTH elemental and shadow.
3. Yes, just because the second swing can curse.
4. Why ? There is no practical difference between AP and Sentenza apart from the damage type. I wouldn't say elemental resistance is way more present in LD.
5. The charge of GF shouldn't be used at all, or only in very specific situations (3 or 4 usable weapons, time to charge).
6. Like every other sword.
7. Obviously, Gremlins call for Shadow damage. See 1.
8. Yes but .... it prevents mobs from riding the charge, and thus lower the charge effectiveness. Combuster doesn't do that.
9. You shouldn't tank JK in the first place. You want a piercing armor, yes, but wolver line have that AND offensive bonus.
10. Didn't try it lately, but I remember JK being hard for 2/3*, especially if you are not well prepared.

In that case, I would go for a Sealed first and upgrade it to Divine Avenger.

You already have an alchemer, and I have never been especially fond of the Antigua-line guns, due to the fact that you need to keep up a constant rate of fire in order to get a good rate of DPS; I find the Drivers to be preferable in this regard, since they're good both on single targets and on clusters of mobs, as well as being able to OHKO turrets under the right circumstances. That's just me, however, and you might want to try them out and make use of the added mobility they have.
You've mentioned that you're planning on getting a Nightblade, so you won't be in need of Faust just yet (unless you want to do RJP runs every single day, in which case, knock yourself out). In addition, the Gran Faust has a ridiculously long charging time, and despite the tooltips, isn't any faster than the Divine Avenger in swinging. It's good for PvP games, but not exceptional in PvE. Gremlins also have a terrible habit of dodging lots, which can make it annoying to try to shoot them down.

Given that you use swords, I would go with antigua and then upgrade it towards being an argent peacemaker. Heck, when I was in your situation that's what I did, even if the circumstances were slightly different.