Sorry that the DL agents couldn’t give you the correct answers, and I agree with others here in their responses:

There’s no complimentary upgrades to D1 on international flights no matter your status or how many seats are left unfilled. They will either be sold to other passengers through upgrade offers (like your offer to upgrade to PS), given to non-revs, or simply left unfilled.

Also, generally once you purchase an upgrade offer, you are not able to further upgrade to a higher cabin afterwards without repricing the whole ticket. That being said I have seen the system offer an upsell offer before where I purchased main, got upgraded to C+ as a PM, and took a very reasonable PS offer. The system did offer a further UG to D1, but it was a very unreasonable price so I passed on it. This was pre-Covid and I haven’t seen any offer like this since. Not sure it would have processed correctly had I took it.

Definitely enjoy the trip in PS, it will be much better than main!

If there are any offers for D1 on the return you should be able to upgrade with no issue if the price is acceptable to you.