Hi CRM,
I do not aim to provide a definitive answer to your question, but I hope the following will nevertheless be of some help.
Having been both an older student and a person who has taught and continues to teach older students (at UG and PG levels) I can understand both your position and (possibly) that of your lecturer. Whilst in my experience mature students do tend to be less confident in their own abilities, are perhaps more demanding of one's time (there's nothing wrong with that), more questioning and are perhaps fixated on things that we see as unimportant; clarity, consistency, encouragement, understanding and solid pedagogic practice can really help to address those 'issues'.
Sometimes, it is helpful to understand that none of this is personal... Your lecturer is probably overwhelmed with work, resides in a state of near continuous chaos and just wants get grading (of far too many papers - why on earth do some institutions still insist on testing the same learning outcomes again and again!) out of the way. They have given you a mark and some feedback in the hope that you will just accept it and you have called them out because you actually want to improve and need more detail and clarity to do that - Something that should be encouraged.
In the real-world, whilst we might have published rubrics, a lot of marking (at least in my subject area - where papers can diverge significantly from expectations) ends up being done by 'feeling'... i.e. this 'feels' like a 70 something percent paper and is slightly better than a paper I've just given 72% for, so I'm giving it 75%. This of course seems somewhat arbitrary, but it is important to point out that that the mark is based upon having seen 1000's of similar papers over the years ensuring some level of consistency. Interestingly, there have been several academic papers on this approach which broadly show it to be no less accurate than following a definite rubric; nevertheless it does make giving meaningful feedback and providing justification for that mark quite difficult which perhaps helps to explain the vague comments about the 'big picture' you have been fobbed off with.
I have no issues with my team using a 'feelings' based approach where necessary (after all, we double mark and our work is scrutinised externally) but failing to spend time with a student who really wants to succeed and is struggling is something that I would not tolerate. Furthermore, being unapproachable is in my book a cardinal sin.
I have no idea how things work where you are, but in my country, we have student representatives and staff-student meetings where issues with a course or individual lecturers can be aired and can (and sometimes must) be addressed. Presumably you're not the only one feeling this way? Assuming that you feel you have exhausted all options with your lecturer directly, is there a mechanism by which your views can be communicated? If so make use of it, you are after all not just a student, but (perhaps - depending where you are) also a 'consumer' and thus have the right to a quality learning experience.
Just my viewpoint.
R