Yes, the rules are scattered. Nevertheless, let's see what we can find out.
A Druid does not gain the bonuses to Strength, Dexterity, Constitution and Natural Armor listed on the Changes to Statistics by Size table. This is because the table is presented in the specific context of improving monsters by HD advancement. It does not apply in other contexts, such as a Druid taking the shape of another creature, or even an Animal Companion gaining bonus HD. It could be said that the Druid gains the "AC/Attack" bonus or penalty listed on the table, but that is because that column is just calculated from a table that is applicable - the Creature Size and Scale table.
The latter table is not restricted in scope as the Changes to Statistics by Size table. If I may dare say it, it is only logical that it isn't - these statistics are as basic as "Space".
Deriving from that table, we get answers to your specific questions:
- No, that table isn't relevant.
- Yes, the Druid's size modifier to hide changes.
- Yes, the Druid's attack, AC, and grapple modifiers change.
- Yes, the Druid's reach changes.
Note, however, that points 2-4 should already be factored into the statistics of the creature into which the Druid is Wild Shaping.
As others have stated, no you can not use a focus (of any kind) while Wild Shaped, even if the animal you morph into has hands, and they are empty and holding the focus. All of the information you posted in the original question provides the answers for you so long as you cross reference them. But a lot of people have missed a crucial part of why it can't be done.
The description of the Druidic Focus says (5e PHB, p. 151):
A druidic focus might be [...] a totem object incorporating feathers,
fur, bones, and teeth from sacred animals. A druid can use such an
object as a spellcasting focus.
This essentially states that you get to create a focus out of whatever you deem worthy to call a focus; pretty neat.
The description of Material (M) components says:
Casting some spells requires particular objects, specified in
parentheses in the component entry. A character can use [...] a
spellcasting focus in place of the components specified for a
spell [but not costly or consumed components]
This is the start of the unravel. Here it says you can use a focus instead of a material component (so long as it has no cost/isn't consumed). So essentially focuses are just fluff/filler for material components that most people over look anyway, but if you don't overlook that stuff it's still a pretty minimal net gain.
Part of the description of the druid's Wild Shape feature says (5e PHB, p. 67):
- You choose whether your equipment falls to the ground in your
space, merges into your new form, or is worn by it. [...]
This portion infers that you get to wear or hold your focus if you so choose when you Wild Shape, which is pretty cool.
The druid's Beast Spells feature says (5e PHB, p. 67):
Beginning at 18th level, you can cast many of your druid spells in any
shape you assume using Wild Shape. You can perform the somatic and
verbal components of a druid spell while in a beast shape, but you
aren't able to provide material components.
This is the crucial spot. It specifically says no material components. If you recall from what exactly a focus is, it is essentially a fancy material component. Meaning it can't be used, not because you can't "hold" it, but because for Game Design purposes they disallowed it.
This is further backed up by the fact that they allow somatic components (hand and body gestures) even if you don't have hands or the appropriate body part to do the somatic component, i.e. a snake making a hand sign. So holding/wearing the material component/focus is irrelevant because it is completely outlawed in RAW.
The druid's Archdruid capstone feature says (emphasis mine):
At 20th level, you can use your Wild Shape an unlimited number of
Additionally, you can ignore the verbal and somatic components of your
druid spells, as well as any material components that lack a cost
and aren't consumed by a spell. You gain this benefit in both your
natural shape and your beast shape from Wild Shape.
Doesn't this seem a bit redundant? It is because it is trying to show you that it is essentially an improvement to Beast Spells. At this point, once you reach 20th level you no longer need the material/focus component. So even at level 20 the answer is still technically no, but it becomes irrelevant because materials/focuses are no longer even needed.
However, this is D&D - so if the DM says, "Well that's stupid, you should be able to use your focus as long as you are wearing/holding it"... Then boom, there you go: the true answer is "yes, you can, so long as your DM handwaves it".
Kind of, with a little wiggle room
I'm assuming your druid is Circle of the Moon since without Circle of the Moon, you are limited to CR 1 wild shapes maximum, per the Beast Shapes table (PHB p. 66):
Circle Forms for Circle of the Moon says:
A circle of the moon druid can transform into a beast with a CR of 4 (elephant) at level 12 and above.
Wild Shape says you use the stat block of the creature you transform into:
An elephant is a Huge Beast according to its stat block. So if you turn into an elephant it is always a Huge one. The wiggle room is that you could probably make the elephant vary in size within the range of Huge for flavor, with DM approval, but it will always be huge. This means that you are subject to the restrictions of a Huge creature.