Regarding double spacing:

In the Old Days, type writers (and even the first word processors) did not add an extra half space after the period to separate the end of a sentence and the beginning of the next one aesthetically. It became convention to leave two spaces after a period and this was the proper thing to do.

But now that proportional fonts leave the aesthetically "correct amount" of space after a period (something between 1 and 2 spaces), it is incorrect to try to force two spaces.

When I use the words "correct" and "incorrect" I mean in the context of conventional writing. It's up to each person if their writing is a little bit more like a poem than prose, in which case they can bend convention as they wish.

As an expert on what is aesthetic -- like everyone else- - I'll comment that the FictionPress font does not provide enough of a gap. I judge the font is going for an old-timey typing-in-the-attic-on-the-back-of-scratch-paper aesthetic; not easy to read but something typers over a certain age might feel nostalgic about.