With regards to "as" conflicting, I think it shouldn't be a problem because the cast operator is binary. So if you already know that something is a declaration, the current token is the identifier that's being declared (or closing parenthesis of the parameter list), and the following token is "as", it cannot be a valid expression. Consider:

  var x as int;

"var" makes it a declaration, so "x" is the identifier, and "as" has to be the type specifier.

  var x = y as int;

"=" clearly separates the identifier from the initializer, and the latter is an expression, so "as" is the cast operator.

Similar reasoning applies to other places where the two can appear - function parameter list, return type, field and property declarations etc.

So far as I can tell, by the time we get to the point where "as" would be used to specify the type of the declared entity, we will already know that it's a declaration, and that the only other token that can follow in existing grammar is "=", "{", or "=>" (for variable and field initializers and default parameter values, property getters and setters, and lambdas and expression-bodied members, respectively), and none of these start an expression. In all other contexts, "as" would be an operator.