Not surprising they are postponing it for the maximum time, again this is all to make you more fed up and more likely to resign.
You could challenge it. Acas website says the following about postponed parental leave;
'If an employer considers that an employee?s absence would unduly disrupt the business, then the employer can postpone the leave for no longer than six months after the beginning of the period that the employee originally wanted to start his or her parental leave. The employer should discuss the matter with the employee and confirm the postponement arrangements in writing no later than seven days after the employee?s notice to take leave. The employer?s notice should state the reason for the postponement and set out the new dates of parental leave. The length of the leave should be equivalent to the employee?s original request.
Employers may be justified in postponing leave when, for example, the work is at a seasonal peak; where a significant proportion of the workforce applies for parental leave at the same time; or, when the employee?s role is such that his or her absence at a particular time would unduly harm the business'.
So if your role is particularly busy at the time of year you want to take your leave, or loads of people are already taking leave then, it could be justified.
I would still sit tight, they are just hoping you will resign if you can't take your time off when you want to.
And I would challenge it, not because you expect to get anywhere, but to give them as much work to do as possible, don't give them an easy life. Also if you can give good arguments as to why your absence will not unduly harm the business, you could at least get the postponement reduced so you can take your leave sooner than 6 months.
Difficult to justify why the business would suffer unduly with you taking parental leave bearing in mind you have been off for a year already and your boss told you he was expecting you to resign...
They obviously know that your request for leave of absence and then your request for parental leave is because you are not ready to come back to work yet. So by refusing all your requests for leave, they are working on the basis that as you don't want to come back to work, you will resign.
So I would consider challenging the parental leave decision on the basis of the criteria above, to keep them busy and go back to work when the time comes as well. If you communicate with them on the basis that fine in that case you will return to work, but will be challenging the 6 month postponement of your parental leave, I think there is a good chance they may crack before you get there, or at least soon afterwards.
Worst case scenario you do end up resigning but I really think it's worth holding on as long as you can - everything you have said indicates that they really don't want you there.