Okihara, understand your predicament and I'm sure many of us have faced frustrations with this sort of thing in the past but I'd like to offer some points to keep in mind. None of this makes up for sloppy Scheduling if that's indeed the case but are things that IMHO and in my experience are apart of any Flight Training School.
It sounds like you're doing your Training Part-Time, this always involves a level of uncertainty that Full-Timers don't quite face as you've got to work around your schedule and that of the School as well. Seems like you acknowledge this already though, Full-Timers are there constantly and able to take advantage of Weather, Aircraft Availability and Instructor Availability far better than Part-Timers or Casuals are, in this way, from a purely business point of view, they'll almost always get priority as they make better margins for operators.
Comparing them to your local barber and his/her scheduling abilities come across as a bit of oranges and apples comparison, also comes across as a bit belligerent of the differences between them, though I get the overall feeling this isn't what you were trying to do I'd avoid it as it doesn't help with your argument.
Let's say they take your advice and commit to N Dates suitable for them, what happens when circumstances such as Weather and Maintenance keep combining to make those dates not happen, who then pays the price and gives up their schedule to make it work? Who also gives up their schedule to make up for the person giving up their schedule...so on and so forth, it's a real domino effect doing it this way. This idea also could potentially stretch things out even further, by doing so it undermines the efficiency of the Flight School as when things keep combining to stop you from being able to do a certain flight on a certain day you might not have other flights available for whatever reason, now the School continuously has Instructors idle which is a massive money loss for them.
The idea of sticking to a couple of set instructors is all about finding someone who suits your style and is designed to give you the best outcomes, not all instructors teach exactly the same despite having the same syllabus and goals, pairing you up with someone who suits your own learning style and personality is a major part of Instructing and should be stuck to as much as possible. I see your frustration when you're there and the Aircraft is there but YOUR instructor isn't and another one is sitting around but that Instructor doesn't know where you're up to exactly, doesn't know your background and could spend half the lesson just getting into a good swing with you which means the overall lesson suffers.
Also as you go further the skills, experience, ratings etc... required to teach you increase, you're now having to deal solely with the Grade 1s and 2s which are a rarer commodity, it often gets levelled out a bit by having more Solo Flights available to you but is not always the case and needs to be kept in mind, there will mostly be a glut of Grade 3s compared to 1s and 2s for a few reasons, one is progression the other is pay level and a third is suitability. Also you make "reading up on theory" sound like a terrible thing which I disagree with, part of being a Pilot is autonomy, being able to find the best ways to suit these moments when it doesn't all line up is a major part of Training and getting things done as fast as possible.
How much you're paying plays a big part, like everyone you probably did your research and determined that where you're at would give you the best bang for buck, but perhaps you're finding now that's not the case, perhaps the place down the road that charged a bit more will have more flexibility, it's like comparing Qantas to Jetstar. Jetstar are the cheaper airline, sure they'll get you A to B the majority of the time at a lower price point but the fact is that when things come undone they have less resources to assist you and it's a lot worse than Qantas with more resources to allocate to getting you back on track. It could be time to look at other options if the School you're at is no longer giving you the performance you expected?
I'll reiterate, none of this is meant to make allowances for sloppy scheduling, if they're not doing the right thing there then there isn't much you can do other that vote with your feet. It feels like your posts are being written out of sheer frustration and a lot could be accomplished by sitting down with someone higher up in the organisation and discussing your frustrations with them and seeing what can be done. Schools can be extremely flexible but if they feel like you're happy coasting along the way you are they won't go out of their way to change that unless you tell them. That would be my first suggestion at this point if you're getting frustrated about the way the scheduling is working.
And I'd suggest it shouldn't be so difficult to swap schools, last I checked they need to make your training records easily available for this reason, grab them, go for a walk around the field and discuss with others what you've been experiencing and what you want now, there shouldn't be any reason to just "stick it out" with them that is overriding to making sure you get the best outcomes, remember this isn't just about getting the Licence, it should be about getting the Licence and feeling like you're really ready for it and that your School has properly prepared you!