The one-string system is something that I am very familiar with, having retrofitted it to 3615. Your one-string line length issue is down to it having been uncleated on one side when pulled on the other, as it is a continuous line through both sides and needs to be fully cleated on one side for it to work, so uncleat the starboard side and pull on the port side to equalise the line lengths before recleating all of the lines.
In terms of the one-string block you are correct in the way that it works. The Shroud tension tightens up the whole system, so if that is fully let off you should then be able to unhook the jib easily, if not then you may need to lengthen the jib halyard. The jib halyard is what is in control of the rake, so you will need to ensure that it is the right length to set your fully upright position and then adjust everything from there. Another consideration as it is a different mast is that the shrouds are probably a different length to the mast that was used to set the boat up with, so you may need to lengthen the shroud purchase. The start point for me would be to get the Jib Halyard sorted, a shackle on the top of the jib may be all that is required, to get the mast upright position set and then work from that.