Poor support for floor Joists

[StressedDIYer]

I’ve purchased a house recently with no issues noted on the survey, we have begun the process of having a new kitchen and when the plumber has pulled the floor up to run some new pipes we have noticed the joists at the back of the house are poorly fitted at the back of the house and not sitting on a wall plate, these are newer joists that look like they were installed with the extension around 30 years ago.

when I look further back at the honeycomb sleeper walls, it looks like the original joists for the house join on to these new joist at the back ,but I also cannot see any wall plates for these joists at all, should I be alarmed?. The house was built in the 1930s would this have been the original way it was built I thought wall plates had been used forever for suspended joist floors?

secondly, it looks like the previous owners have broken through the honeycomb sleeper walls to give themselves crawl space access to run cables and new pipes and left the bricks out for access. The joist seem to not be bouncing or moving and as I said no issues on survey should I crawl under and reinstate these walls or is this common?

Lastly (sorry for long post) the joists are uneven can I pack a few of them slightly to level things a bit or is this a big no no?

Thanks