I don't blame you. My experience is that turnover of heads/senior leadership turmoil like that suggests that the school itself will be in disarray. You might have great teachers within the school but without leadership and vision it's not the best, especially for children with additional needs and ones as rare and poorly understood as SLI (even ASD, which a lot of people think they know about).
On the other hand, the best unit I ever worked in was in an Ofsted school rated "satisfactory" (though the unit was deemed outstanding, this wasn't on the public inspection report) so I would investigate it further as although it's not ideal, units can still be valuable if the team within the unit is well managed and well run. If not, well.. that's a different story.
3 hours is a lot, don't underestimate it. It would be 2-3 hours from the SLT, 3 hours carryover from a specially trained assistant and then 19 hours of in class carryover. Think of the SLT as being like your consultant - setting and reviewing progress on programmes. Very few ABA programmes would have that amount of weekly input from their consultant.
In my units, the 3 hours I spend with a child would basically be me working with them on the programmes that have been set trying to increase their success - Like in ABA teams, every team member would feed back on what's being going well etc and what needs support. The assistant who carries over would be in this session too and we would work together. I would model, she would try it out, I would tweak it, we would agree on the goals for that week's work which would be reviewed next session. I tend to do one which is a social group in my settings too so 2 individual plus 1 group. All of these programmes are repeated while I am away at other settings and I am in daily email contact if staff need support. The support staff in class have all had training and also have responsibility for carryover so act as keyworkers for particular children.
Not all units work like this. I don't think mine is gold standard, there is a lot I would like to improve. I woud like ideally to have all of this AND have the opportunity to go more into class and ensure that the provision is at a quality I would like from my support staff as this is where they spend most of the day.. I would particularly like to kick some teachers up the bum who just leave everything to support staff. I have one setting where there is good commitment to whole school support of the unit's approaches and where a cohort of teachers have committed to weekly ongoing speech and language training and act as champions within subject areas for the whole school, but in that school, I haven't really got as good a set up for individual and group work as the resources were allocated more for training than for individual students?
The ultimate unit (if one exists?) would ideally have a tightly run unit with support from the senior leadership team to truly integrate students at all levels, with intensive SLT support AND carryover for all students. Unfortunately, very few settings have full time SLT support these days. When I started, I had half a week - now it's a day and a half - and the students' needs have become more extreme. I used to have the luxury of going around the support staff modelling in-class support as well as doing more individual and more group. I don't now.
Evidence-wise, there just isn't enough in terms of dosage for you to really fight for more than this. If you can get an SLT to write a very tight report that suggests a high degree of individual is needed and is very careful in terms of wording specifying level of training/duration of input etc etc, you may have some luck but unfortunately those sorts of statements are really difficult to procure these days even where there is a much stronger evidence base.
I hope this doesn't sound too down-hearted, just trying to be realistic.