Keeping to the numbers
0. Need to see upstream figures to get the full picture
1. PlusNet do not have anything to cap connection speeds, only the BT Wholesale system, and that may apply banding if it thinks the line is unstable but connecting at similar speeds all the time.
2. Yes it can, but if you can change it with your modem then DLM is not doing that
3. Magic word, is can you reset DLM as it may be banding has been applied to the line.
4. If you want, but generally delays any improvement from the automatic system
5. They can see line history data. The 448 Kbps upstream is an oddity you can ask for that to be unlocked and will probably double.
Generally takes two weeks of stability for DLM to start handing speed back to you.