The substantial question 2 being already answered, I will address the less substantial 1.

Even if not substantial, the formalization of 1 might not be immediate (and at the end all the question might be considered by someone only a logical sophism, but since questions of the kind "is a canonical construction available?" arise naturally,I will answer anyway. In some cases the answer to such questions depends upon subtle corners of the formalization).

First note the following formal answer: yes, take as independent every (nonempty) subset of $E$.

It answers the question as posed (it is even the "largest" matroid extending $M$ on the same base set $E$),but it is clearly an undesired answer. It is a matroid generally too far from the initial independence system.

So let us search a "minimal" matroid extending a given independence system (minimal for set inclusion; by the substantial answer to 2 the independence system is the intersection of such matroids).

If the base set $E$ is not only a mere set, but a "list" (a finite set equipped with a total order), then again an (again unwanted, trivial) answer is available: take the first minimal matroid that extends $M$ (the first for the the "natural" order on $P(P(E))$: note that if $X$ is a totally ordered finite set, then so is also $P(X)=2^X$ with the lexicographic order).

This is again (formally correct but) unwanted since (a) no intelligent construction of the matroid is given (only a brutal "try each axiom and see"); (b) the answer depends on a (unrelated with $M$) total list order on $E$.

So one might request a request at least independence from the listing order.

Really, in general, in questions like this (can I "canonically construct" a structure of type $A$ on $E$ starting from a given structure of type $B$?) one requests "functoriality" (concrete over the base set $E$) at least for isomorphisms; in particoular, the group of transformations of $E$ that are $B$-automorphisms must also be $A$-automorphisms.

[This must be so for any kind of reasonably first order definable constructions; see Hodges, model theory. In this specific case, the structures are not directly given as first order ones, but one can cripto-equivalently define them as first order structures with a standard trick: in place of $E$, take a atomic boolean algebra (so that $E$ is identified with its atoms); a added unary predicate "independent" applied to elements of the boolean algebra permits first order translations of the axioms .(using in place of 3 the form of the exchange axiom that works also for infinite dimensional matroid lattices: the covering property)]

With this "functoriality" request to formalize "canonical", an easy example shows that in general a canonical construction is impossible:

on a set $E$ with three elements $a,b,c$ take as maximal independent sets the singleton of $a$ and the pair of $b,c$. To minimally extend to a matroid one has to take as basis besides $b,c$ also either $c,a$ or $a,b$.

(think $b,c$ as a horizontal and a vertical vector in the plane. As $c$ one takes either another horizontal or another vertical vector; non minimal matroid extensions take as $c$ a oblique vector in the plane or a vector outside the plane).

The group of automorphisms of the initial independence system contain the permutation that exchanges $b,c$ (and no other non-identity element). This exchanges the two minimal completion instead of being an automorphism for one of them, so the impossibility of "functoriality" is obtained.