Learn English – What to call a certain shape of lips

"We'll be coming back" refers to the proposed meeting at "half ten" (9:30) and says what the speaker expects to be doing at that time. (Future progressive tense.)

"We'll come back" states their return as a fact, and nothing else. You might use this if you don't know when or how you'll come back. (Simple future tense)

"We're coming back" is only slightly different from "we'll come back" in this context, but it makes it clear that there is a plan. (Present progressive tense) It treats the trip out and back as a single continuous action which is already in progress.

"*We'll coming back" is not English. Don't say this.

As far as the chart you reproduce is concerned I think it's misleading, there's no difference in position between beneath and under and both should be on the same level.

I would say that beneath and under are near synonyms, they both mean that something is lower than something else. The essential difference is in register. Beneath is more formal, more literary. The choice of one over the other depends on what you want to convey.

I'm wearing a sweater under my jacket.
I'm wearing a sweater beneath my jacket.

both mean that the jacket is on top of the sweater.

Some people might see a difference between:

(1) The ground under my feet.
(2) The ground beneath my feet.

1) referring to the actual piece of ground my feet are touching
2) referring to the ground in general, or a larger piece of ground than just the actual piece my feet are in contact with.
I would say it is context dependent and I would use beneath in both cases, and maybe not under if I meant the ground in general.

When used in an abstract way beneath is preferred to under.

• Myrtle Wilson (in the Great Gatsby), married beneath her class.
• If I find that something is beneath my dignity, it means I have such a high opinion of myself hat I could not do it without degrading myself.

But:

We have a lot of work at the moment and we are under great stress.

Note:

As in you examples I have only considered the use of both words as prepositions. But note that under is very rarely used as an adverb (I can't think of an example where it would be used as one - which does not mean it is impossible, of course) whereas underneath can.

The garden is just under/beneath my bedroom window. (preposition - it is followed by a noun)
When I look out of my window I can see the garden beneath/underneath. (adverb)