I think the backslash \ is the correct solution for hard line breaks and it should be the only option.
Please, get rid of the two-space line break in the spec. Don’t encourage people to use that abomination! To the obvious argument that you cannot edit what you cannot see, I’ll add that some editors remove trailing spaces in lines as a convenience (mine certainly does).
Regarding 6.10 Soft line breaks, I think it needs to be clarified that a conforming parser will not convert them to <br> by default.
Thanks for this great effort!