I don't think that the questions have definite answers yet, but I think they are great for general discussion.
I am addressing the second question related to the AdS/CFT, and I am quoting -copying and pasting- some known facts from a recent paper. In case one wants to study all the details of the paper, find it below.
https://arxiv.org/abs/1712.08570
and the references therein of course; in particular the eight first ones.
People are trying to calculate beyond the SUGRA corrections to M-theory, in order to get some first lessons on this theory
M-theory arises in the strong coupling limit of type IIA string theory.
It is known that M-theory reduces to $11$d SUGRA at
low energies, and its fundamental degrees of freedom are $2$d and $5$d objects known as M$2$-branes and M$5$-branes, respectively.
The worldvolume theory for M$2$-branes (on an orbifold) was found to be a $3$d superconformal Chern-Simons matter theory with classical
$\mathcal{N} = 6$ supersymmetry.
The M$5$-brane worldvolume theory is expected to be a $6$d superconformal field theory with $OSp(8_{*}|4)$ symmetry; this is usually called the $6$d $(2, 0)$ theory.
If one considers a single M$5$-brane, one can formulate the theory in terms of an Abelian $(2, 0)$ tensor multiplet, consisting of a self-dual $2$-form gauge field, $5$ scalars, and $8$ fermions, but it is not known how to generalize this construction to describe multiple M$5$-branes.
This is where the AdS/CFT enters the game to give a hint. The statement is that the worldvolume theory for a stack of N M$5$-branes is dual to M-theory on $AdS_7 × S^4$ with N units of flux through the $4$-sphere,
which reduces to $11$d SUGRA on this background in the limit large N limit; $N \rightarrow \infty$.
The above limit is the SUGRA approximation.
Recently, people have started thinking ways of going beyond the SUGRA approximation, by using the conformal bootstrap programme. This is how people hope to gain new insight into M-theory beyond the SUGRA approximation.
It is elusive yet how one can formulate the $6$d $(2, 0)$ theory, and this is why people are using the superconformal and crossing symmetry to deduce the structure of the $4$-point correlators in this theory.
The implications of the aforementioned symmetries for the $4$-point
correlators of stress tensor multiplets in the $6$d $(2, 0)$, adapting the holographic arguments, have showed that these solutions scale in the large-N limit. The spin-$0$ solution scales like $N^{−5}$, all higher-spin
solutions are suppressed by at least $N^{−19/3}$. As a result, the spin-$0$ solution corresponds to the leading correction to the supergravity prediction for the $4$-point correlator (which scales
like $N^{−3}$) and all higher-spin solutions are subleading compared to the $1$-loop supergravity prediction (which scales like $N^{−6}$ but has not been computed yet).
The lessons are about the low energy effective action of M-theory on $AdS^7 \times S_4$. In particular, noting that the anomalous dimensions of the spin-$0$ solution scale like $n^6$ compared to those of the SUGRA prediction (where n is the twist), this suggests that the corresponding terms in the effective action contain six more derivatives than the SUGRA Lagrangian, and are subsequently of the form $\mathcal{R}^4$, where $\mathcal{R}$ is the Riemann tensor.
Edit: M-branes and D-branes.
Punchline: The M-branes realize all D-branes, and this is why D-branes are consistent objects that you don't need to throw away.
Eleven-dimensional supergravity has only an anti-symmetric tensor field $A_{(3)}$ -the rank of the tensor is 3- and the realisation of branes is restricted.
The consistent supergravity solutions are known as M$2$ and M$5$. The are further solutions to classical supergravity known as F$1$ -the fundamental string- and its magnetic dual the NS$5$-brane.
You can classify the theories based on the type of supergravity theory you get in the classical limit.
The way that M-theory sees D-branes is via the net of dualities.
All of the D-branes and the NS$5$ brane are solutions to type II theories, both A and B.
When you reduce the M-theory on a circle, you get back to TypeIIA. TypeIIA is the same as TypeIIB when you perform a T-duality. The M$2$ and M$5$ branes reduce to the various D-branes and S-dualizing the D$5$ brane you get the NS$5$. I don't remember the actual dualities you need to apply, but you can go to the seminal papers and look there for more detailed answers.
