Data structure for sparse matrices for an online problem
In low dimension, Seidel's algorithm can be useful: if we have the optimal solution to $m$ constraints in $d$ dimensions, and you add one more constraint, then the amortized cost to find the optimal solution to those $m+1$ constraints is $O(d!)$, assuming the constraints are being presented in a random order.

(It is usually presented in the following form: if we have $m$ constraints, and we add them one-by-one, updating the optimal solution after each constraint is added, then the total running time to do this is $O(d!m)$ using Seidel's method.  But that's equivalent, if we assume constraints are added in a random order.)

In high dimension, I don't know of any good method.  There is a trivial observation that if $x^*$ is the optimal solution to the constraints, and we add a new inequality, and $x^*$ satisfies this new inequality, then $x^*$ remains the optimal solution, so there's no need to re-solve the LP.  This condition can be tested in $O(d)$ time.  However, if $x^*$ doesn't satisfy the added inequality, I don't know if there's anything better you can do than solve the new LP from scratch.  Same for changing or removing an existing inequality, or changing the objective function.



I'm not sure your question about data structures is well-defined.  Data structures are chosen based upon the set of operations you intend to perform on them.  In this kind of problem, it's usually the algorithm that is the more important part.  You start with the algorithm, then choose a suitable data structure.  But my impression is that with LP, the hard part is the algorithms, and the data structures tend to be comparatively simple.  So asking about sparse matrix data structures  for your problem seems to be putting the cart ahead of the horse.