You can, and arm CLB/OPEN CLB then, mean you'll perform the acceleration from then on.
I am not sure what you mean.
Yes, you can prematurely select CLB thrust, but this will not affect the acceleration altitude. In fact it is routinely done in order to lower the climb rate whilst staying in SRS mode.
Once in SRS the only way to engage OP CLB/CLB mode is by:
-passing accel alt
Or
-pulling or pushing the ALT knob respectively.
The Thrust Levers do not interact with the vertical guidance modes, except for engaging SRS when TOGA is selected and Flap Lever -< 1.
2/. What happens to the speed if you stop climb - when a/t armed and a/p in, whilst you are level? (Managed climb - but managed climb would not be managed if you stopped climb?)
Not sure that I understand the question.