You probably meant ∑ (\sum) rather than Σ (\Sigma). The two are different symbols, and they are sometimes used in the same expressions. For example, in the field I (used to) work, a sum over all characters in the alphabet would be written as \sum_{c \in \Sigma}.

Mathematical syntax is complex, and the typesetting often becomes unreadable outside trivial cases. Using Unicode won't help much, especially because it's easy to confuse similar-looking but unrelated symbols. Proper syntax highlighting might help, but I've never seen a tool doing a good job with it.

The first obviously-better idea is to use a/b instead of \frac{a}{b} and (x) instead of \left(x\right).

The next obvious improvement is to allow (not require — allow) me to write α instead of \alpha, ∈ instead of \in, and ≤ instead of... whatever the code for that was.

I feel that the answer to your question is right there in your question.