I'm a solicitor who is just few years older than you, but have been qualified for 30 years. I've been a partner in 2 law firms, so I have some experience of this.
I haven't read the full thread, so apologies if I echo anything that has already been said.
I can't comment on market rates for newly qualifieds in your area, but £27K does not sound unreasonable. I would certainly first check whether or not it is in the right ballpark; you should be looking for the very top end of the NQS band, or even higher/with quick promotion prospects, given your pre-qual experience.
You then need to look at the role that you are being offered, because, as others have pointed out, it is very far from a typical newly qualified role. They are asking you in effect take on a substantial proportion of the management of the firm. Are you able to work out roughly what percentage of your working week that will take?
If you try to keep on/expand your caseload and do a substantial amount of non-chargeable work, you will quickly burn out and you will also be very badly underpaid. If you agree with them that you can work a reduced caseload, and that billing expectations from you are lower, to accommodate a large proportion of management work, then in effect, you will start your career as a solicitor as an office manager and I'm not sure that I would want that. Do you?
I think you need to decide whether this is what you had envisaged. If you had not, and you don't want that type of role, then you need either to be having a serious talk with the firm to see if they can give you the sort of role as a qualified solicitor that you want, or you need to look elsewhere. You would be a great catch for another firm who is looking for a fee earner who is newly qualified in name only.
I would have no truck with any suggestion that you owe anything to the firm because they paid to train you. It sounds from your OP as though the leadership role that they have proposed was not something they discussed with you at the time that they were supporting you to qualify.