I am a speech pathologist in Texas. Big hugs to you, mama. I've seen a lot of families struggle with this and being "speech only" but everyone knows its more than that.
first of all, RTI is a federal thing now. so its not just Florida. It does take a long time, but it is a MUCH better system than before when kids qualified as LD based on discrepancy between IQ and achievement scores. The old system seemed IMO to often identify those children who had issues at home, behavior problems, etc. that caused gaps in learning. If you had a lower IQ, it was almost impossible to do poorly enough on the achievement tests to show the required 16 point spread. RTI looks at how a child does when given research based interventions. If they fail to make expected progress, then you can reliably say it is not just a gap in instruction. Its a good system, but it takes time to gather that data. Where I work (in a highly mobile low income situation), it is almost impossible for us because the kids move before the data has been fully collected. argh.
Why is your daughter supposedly having to be retained? Classroom grades? Standardized test? Will she attend summer school?
In our district, we have to have an ARD meeting (might be called IEP meeting in Florida) on all children who are in special education (which includes speech students). The ARD/IEP committee is considered the grade placement committee. It is strictly up to that committee to decide. I believe this is federal law (that the ARD/IEP committee is the grade placement committee).
Therefore, for the reasons you've stated, the committee can agree its in your DD's best interest to be promoted to 5th grade.
I do not know about Florida, but in Texas 5th grade is a special year. If they do not pass the standardized test in both Math and Reading, its automatic retention

Again, the IEP/ARD can override that (and should in most cases) during the ARD.
What it comes down to is that more than a few retentions ends up being a problem later (when you have a 14 yr old in 6th grade). Then they do whats called a "bump up" and place the child up a grade. If that is done, a child can NEVER qualify as Learning Disabled EVER (because they've essentially missed a year of instruction. its ridiculous).
When was the last time your DD was tested for a learning disability? Was it under the old rules (16 point discrepancy) or with the RTI data? If it was the first way, I'd really try for that evaluation now. She might qualify under the new regulations. It sounds like they are collecting RtI data now on her. Is this true? How is she doing so far? What kind of RtI are they doing?
My guess is that the administrators are as confused on this as everyone else is. It seems to change from year to year.
The bottom line is that the ARD/IEP committee is in charge. The law says that a child in special education needs accomodations and support so they can be successful. If any of my students are failing or struggling, I am supposed to have an ARD/IEP to discuss what we are doing to address that. If they need more accommodations, we should be providing it. Special Education should be a safety net for these kids. The difficult part is that I need to also be sure I can relate my accommodations back to the diagnosed disability. In a lot of cases, "speech only" doesn't give you enough to put in major curriculum changes or modify curriculum.
I would see where you are in getting another LD eval. And I would ask the ARD/IEP committee to consider her grade placement in 5th grade based on progress she has made this year, what would be in her best interest with her self esteem/confidence, etc.
Before you do that though, make sure you tell the ARD/IEP members ahead of time what you want. Even involve a higher up (special education coordinator, program director). You might be surprised to find out the school is actually wrong and the program director will set them straight. Tell them you'd like the name of the director and a phone number to reach him/her. In a polite way of course. I'd just say "As her mom, this issue is very important to me. I want to speak to a special education program director that is more familiar with special education law than either you or me and see if there are any other options we haven't explored." If you get into the ARD and then bring it up, they won't know what to do (if its against their standard rules) and it will probably not go somewhere. I'd spend a few hours on wrightslaw.com and get some good documentation about ARD/IEP committees making grade placement decisions and about retention of students in special education.
I am in favor of your daughter being promoted to 5th given the info I have in your OP.
If I can be of any help, please let me know. I'm happy to even chat by phone. I might confuse you (but believe me we are ALL confused on this crummy issue).
XOXO
B