RAW
No damage dice => No dice to double
Sucks, but them's the rules. So a critical hit with an unarmed strike does nothing more than 1 damage, excluding modifiers. (I'll be excluding those for the rest of the answer) The tables in the player's handbook doesn't list a "1d1", just 1 damage. "1" is not a dice roll, it's a number.
Of course, the designers do seem intent on making unarmed strikes do 1 damage. I doubt it's a typo, especially considering the basic rules have been updated (it's on v0.2). They even supplied a feat (tavern brawler in PHB) and a class (monk) which increases the unarmed strike damage from 1+mod to some actual dice (like a 1d4). If they never intended the damage of unarmed strikes to be so lame, why would they provide that feat and that class?
As a side note: a critical hit from any weapon causes two failed death saving throws... including unarmed strikes. This is possibly the only time they are on par with more traditional weapons.
Possible RAI and RAF; The Spirit of Critical Hits
Critical hits seem to be meant to do more damage than a normal hit. This is reflected in the doubling of an attacks damage dice. It makes you generally do more damage than normal, although there is a slim chance that you don't do as much on that strike. You can see the odds for some weapon dice values here.
If you're looking to extend this logic to critical hits, I suggest instantiating a house rule for upping it to a 1d4 or supply 2 damage on a critical hit. This could be a fun rule, and it may alleviate the disappointment your players feel while getting that critical hit while hitting something with their... fists? ("Unarmed Strike" is really vague!)
However, it does seem that the designers really did want unarmed strikes to not hit hard unless you've had some experience. Once again, this is because of the Tavern Brawler feat in PHB and the monk class. Is this because they want people to use weapons unless they're a monk or a tavern brawler? Were they afraid of things like the grapplemancer builds coming into 5e? Hard to say. You could ask wizards of the coast by emailing sageadvice@wizards.com.
A Note
5e greatly acknowledges the importance of house rules in D&D culture, so I would not stick with the RAW reading of this. In that article linked above, written by Jeremy Crawford, one of the two lead designers for 5e, says:
We expect DMs to depart from the rules when running a particular campaign or when seeking the greatest happiness for a certain group of players.
The RAW just is not fun and takes the joy out of a critical hit. Come up with a rule that seems right (doubling to 2 damage, or going to 1d4 damage, or something else) for your group. In the words of Jeremy Crawford:
Fifth edition now belongs to the thousands of groups playing it.
Modify 5e to have fun in your group!
From the PHB (p. 196):
Critical Hits
When you score a critical hit, you get to roll extra dice
for the attack’s damage against the target. Roll all of the
attack’s damage dice twice and add them together. Then
add any relevant modifiers as normal. To speed up play,
you can roll all the damage dice at once.
For example, if you score a critical hit with a dagger,
roll 2d4 for the damage, rather than 1d4, and then add
your relevant ability modifier. If the attack involves other
damage dice, such as from the rogue’s Sneak Attack
feature, you roll those dice twice as well.
The example given answers your first question perfectly.
For the spider, the secondary damage is from poison, so is not inherently part of the attack. This can be seen by the fact that the damage doesn't depend on the attack roll, but a separate saving throw.
Critical hits represent hitting a vulnerable area. With poison, it doesn't really matter where you are hit.
Best Answer
This interaction is the same as Sneak Attack.
Sneak Attack:
Divine Fury:
Note the key phrases "extra damage" and "creature you hit". The abilities work the same way, so there's no reason to think that Divine Fury's extra damage isn't doubled on a critical hit. There's no language that indicates triggering of a separate effect, like "when you hit with a weapon attack, you may ... ". It simply says that your first weapon hit on a turn does extra damage.
As far as the issue of Divine Fury having (as phrased in the question) been "changed from an aura", that simply isn't relevant. Unearthed Arcana material isn't final or official, and sets no precedent. It's misleading to think of differences between UA material and the contents of XGE as "changes". UA is an incomplete draft released for testing. XGE is the actual rules, not a revision or rules change.