My understanding is,
1) If I get a mesher order, whereby he has to pay the mortgage until our child is 18, will this count as SM and be deducted from UC?
Like others have said, the mesher order wouldn’t make him pay. It’d just let you live there. You’d need to be able to pay it.
2) If I can get £850 on UC, plus say £450 CM from him, that puts me on £1300 a month. (Maybe more if I could find work). So would a judge ask me to pay the mortgage myself out of that? (which is between 3 and 400).
Yes, you’ll need to pay the mortgage.
3) If a judge says I must pay the mortgage myself based on point 2, but the mortgage company says sorry, you're not in a job, or earning enough, we're not naming you on the mortgage - do I still have to pay it?
Yes, if you want to live there. Or it’ll be repossessed, and you’ll be evicted. The judge cannot make the bank name you on the mortgage, and the bank won’t do that unless you pass affordability checks.
4) If judge expects me to work, that's still not the same as 'ordering' me to work, right? As I say, you can't guarantee a job opportunity or successful application. Or job security come to that.
No, it’s not. It just means they won’t fund you. The reality is that UC is legally what you need to live on, so if you want more than that, you’ll need to work.
I’m sorry that this seems to have come as such a shock, but I think you need to prepare yourself for the fact that you’ll need to work to maintain a higher standard of living. It’s pretty tough for single parents.