Thanks. You helped solve the problem: I went against your advice (out of exasperation) and installed the appropriate stable version of CURL.
CURL installed correctly on my WIN10 64bit machine and runs fine.
I was able to use CURL to upgrade the Marvel License following curl'ed instructions.
(P.S: I also installed REST prior to installing CURL: had same problems with all the browsers that I reported here earlier)
So this particular problem was solved by sticking with CURL.
(I have since run into another problem with WIN10 and bootstrapping with syscall which is preventing ES-Marvel from starting up--see my latest problem post).