First problem:
Standard length shrouds are now shorter than they used to be due to the desire for more mast rake and the redesigned jibs. As a result, standard length forestays are now longer.

If you are happy with the mast rake you had with your old shrouds, you may be able to acheive it with the new shrouds. The important thing is the mast rake geometry and not which hole in the side chainplates you are using. Put the old shrouds on and raise your sails as usual. Using the main halyard to measure, mark it at a convenient reference point on the boat (lip on hull at the transom, for example). Put the new shrouds on and raise your sails again. Pin the shrouds in the chainplates such that the halyard mark meets your reference point. See if you can pin the forestay to the top hole in the bow chainplate(s). If you can, you are done. If you cannot, the slick solution is a longer forestay. The "redneck" solution is a daisy chain of additional shackles, chainplates, what-have-you.

If you want and can acheive more mast rake, it may be limited by how tightly you can sheet the jib (and the forestay length problem).

Second problem:
I don't understand why you are messing with the side chain plates (walking down the pins with a screwdriver, etc.) The side chain plates are typically "set it and forget it". Rig tensioning on a H16 is accomplished by the jib halyard. After raising the mast, pin the "standing" forestay to the top hole in the chainplate (many folks use a double chainplate). Attach the jib and then tension the rig. The internal forestay in the jib becomes the "working" forestay.

Please let us know your results.