Coming back questions.Follow

I can't personally speak on the acquiring of gear from 1-99 since the majority of my jobs were leveled well before some of the major changes to the game were made, but there are readily obtainable gear options from Abyssea (which, while accessible from lv. 30, is tailored towards characters starting in the mid to upper 70's) that can serve you until reach lv. 99 and gain access to Adoulin gear and the like. There are several lv. 99 (with varying item level) weapons and armor that you can obtain through Skirmish, Wildskeeper Reives, Records of Eminence, and Bayld that you can at least having a working set of equipment to use in a good deal of end game content so you can work towards some of the harder to obtain/more time consuming gear (item lv. 119 Artifact/Relic armor, Divine Might, Arch Angel/etc hard mode weapons, etc).

For job combinations, based on lv. 99 content, so there might be some different SJ options at lower levels due to when you'll get traits/abilities, it depends largely on what content you are doing. Subbing Ninja is still used in fights like hard mode Divine Might because of the vast damage output of the Ark Angels, but in general Dual Wielding isn't as often used as before. Samurai is basically /War or (very rarely) /Nin, and most other two-handed DD (Drk, War, etc) will use /Sam due to Store TP for X-hit builds, Meditate, Hasso, Seigan + Third Eye, etc. Dual Wielding axes still has its place for War here and there should you have the Ruinator merit weaponskill, so /Nin is still an option for that too. Drk, in situations of pure offense, will sometimes go /War in place of /Sam, as their high delay weapons and the "recent" buff to Last Resort, help to make up from some of what they're losing from /Sam, as well as gaining Berserk, Aggressor, Warcry, and Double Attack.

Pld/War is still common, as is Pld/Dnc for situations of needing to tank a monster away from a healer and needing to be self sufficient in terms of status effect removal. Nin/War is also still very common, but /Dnc is again a good SJ as well, especially for soloing. It offers you a way to cure yourself, additional ways to enfeeble enemies via Steps, a decent Stun from Violent Flourish, and, most notably, Haste Samba.

For Black Mage and White Mage, /Rdm has pretty much become the new standard subjob, as it gives them full potency Refresh, Convert, Phalanx, and Fast Cast, while still giving Blm just about everything they got from /Whm, barring AoE heals and -na spells (which Blm aren't going to often need to cast anyway). Scholar will also use Rdm for most all situations (for the same reasons given for Whm subbing it, as well as being able to Accession bar-spells, Phalanx, and en- spells), except for when they're needing to Stun, in which case they're going to go /Blm. Sch, thanks to its various Grimoire abilities, has the most accurate and quickest casting/recast Stun in the game right now, making them the best stunner in those situations where it's necessary. Rdm will often be either Blm or Whm sub, depending on the situation they're needed. Smn will generally use /Rdm for all but the rare situation where they know they'll need to remove debuffs too often to rely on Bloodpacts, in which case they'll go with /Whm.

Thf now has access to natural Dual Wield and, at 99, has access to the same potency of Dual Wield as it'd get from /Nin, so the only reasons to /Nin are Utsusemi, Sneak/Invis, and as a proc trigger for things like Abyssea. Otherwise, they'll generally go /Dnc or, for purely offensive purposes, /War.

Mnk is a job you didn't mention that has become quite a major powerhouse again, and outside of the situations where it needs Utsusemi (namely those hard mode fights, etc), they're /War.

While some of the higher-tier content (namely the Hard mode versions of fights like Divine Might, Stellar Fulcrum, etc) are catered towards full party/full alliances, a lot of content now is tailored for more casual, low-man situations, where a handful of people can go in and make quick progress. Most Skirmish fights only require a handful of people, and each run is over in half an hour at most, and that's the general sort of change to playing the game. It's better suited for quick progress with smaller groups rather than gathering up alliances and taking several hours to do something (like old Dyna used to be). Usually those larger fights are either LS based or formed with pick up groups.

Grounds of Valor (GoV) is essentially Fields of Valor, but in dungeon areas rather than out in the field (Gusgen Mines, Crawlers' Nest, etc). Grounds of Valor has benefits over Fields of Valor, in that the actual exp per page completed of the same trial is increased with subsequent completions up to a cap. For example, page 1 might start out with 600 exp reward, but doing page 1 again will result in an exp reward of 700, and so on and so forth until it's 2100 exp per page. Again, I can't attest really to what others are doing since I'm not exping anymore, but generally people seem to not bother with Abyssea exping anymore in favor of leveling through GoV pages and using Trust NPCs to act as supplement players in a party. The new Trust system lets you summon multiple existing FFXI NPCs to aid you in battle, acting as DDs, buffers, or healers, depending on each (Shantotto's a Blm, Ulmia's a Brd, Zeid is a Drk, etc). They're very hardy and reasonably intelligent in terms of AI, which lets you kill things with considerable ease and rack up exp quickly.

Some jobs have become far more sought after than before, while others are now or are still relatively unneeded/unwanted. Red Mage for example has become far less desired than before, namely due to its strengths in the roles it was called for becoming overshadowed by other, better options . Puppetmaster, Dragoon, and Beastmaster are also ones that aren't really wanted, though part of that is due to continued persecution despite several advances job adjustment wise that have made them far more formidable. Recent changes to elemental magic has actually made Blm very strong in general, for the damage formulas and cast times/recast times have been changed to where you can readily spam tier I spells for significant damage. This was done to counter to help even out the distance in damage output that existed between melee DD and Blm.

I know that doesn't cover everything you've asked, but that should hopefully help a bit in terms of catching you up to speed on some things.