I can see how difficult it might be to answer specific placement questions when dc have been in ps for a few years. As far as R & S English, starting at 5 would be just fine. You could certainly start that now, and just do a lesson a day, should you so desire. By the time dd graduates, she'd probably complete R & S English 7, which would be more than 'enough.' Grammar is not technically even required in most high schools, and R & S English is advanced already. By starting with 5, dd will be able to pick up on the diagramming and move forward nicely.
As far as overall placement, for 8th grade it would seem you'd be choosing between RevtoRev and MTMM. For 9th grade, it would seem you'd be choosing between MTMM and WG. In general, not knowing much about the specifics of dd's placement, I think RevtoRev or MTMM would be good to do first. If she can do MTMM for 8th grade, I would think she'd be able to do WG for high school. If she doesn't do 8th grade and starts in 9th grade, I'd probably have her do MTMM for 9th grade. With my ds Wyatt, RevtoRev and MTMM seemed to be somewhat lateral - quite similar in difficulty and not a huge amount of difference in the workload. The change from MTMM to WG was greater. The level of difficulty of reading across all subject areas, the length of readings, the length of the school day, the length of pages read dc are orally narrating upon, the level of expected independence, and the amount of writing all increase. Reading the classics and doing BJU Lit for WG, for example, is harder than doing DITHOR 6/7/8 for Intro to Lit in MTMM. The 5 to 5 1/2 hour school day of MTMM (this is without extensions though) compared to the 7 hour day of WG is a change as well. The science lab in WG is quite difficult and doing MTMM's science prior to it certainly helped. Having a 2 year break (if I'm reading that right) in practicing and increasing the skills of narrating - both orally and written - assuming these skills were not taught in ps, does make a difference in placement too. I am hoping something here can help as you consider placement for dd! HTH!
In Christ,
Julie