Your boat looks really nice and clean!
I'll help as much as I can
I hope others with more experience will correct any of my mistakes!
I have a 21SE with a spinnaker that I use most weekends.
Having experience with other cats and spinnakers, I wanted to induce pole bend. I tie the spinnaker mid-pole to the bottom of the roller furler with a shackle that can spin. This also replaces the orange lines in your picture. I have seen pictures of other boats that use other methods to pull up the middle of the spin pole. I like to see about 2" of prebend in the pole when it is sitting static, and you should be able to lift the bows with the tip of the pole, without the pole going fully straight or inverting. I use amsteel line from the bows to the front of the pole. You can tension the pole with the bow lines using a shackle at the bows to give you a 2:1 purchase, but I find it easier to find the correct length, then tension the mid-pole to the furler. (Edit) I see that you have blocks here, you can run your line from the pole to the block, and back to the pole giving you the purchase you need to bend the pole. Use a very low stretch line for this.
I had the same halyard setup that Hobie shows in the drawing before I tensioned the pole, but I didn't like the upward pressure on the foot of the pole, so I separated the tack and the halyard, and moved the halyard lock (auto cleat) to the mast. The crew likes it this way as it is easy to pull the chute to the end of the pole and says it is faster to pull down on the halyard to raise the chute. either way seems to work fine. Your pictured method removes one step in the process...
As to the tack line length, it takes a little trial and error, but you just need to make it short enough to pull the tack to the end of the pole before the double block gets to the block at the foot of the spin pole, and long enough to put the tack in the bag. The halyard just has to reach the double block and raise the sail fully. Your adjustment is really only the tack side.
The 21 has a really big spinnaker, it makes it tough to use a pole snuffer, so most everyone just uses a bag on the tramp. Mine has carabiners on the bottom to connect it to the front of the tramp. While the skipper controls the halyard and sheet, the crew grabs the foot of the sail and stuffs the whole mess into the bag on takedown. To launch, open the bag and pull the halyard\tack after the skipper grabs the sheet.
Two spinnakers: lay them out and see if they are the same size, I understand that the 21SE has had two different chutes, a small one originally, then the much larger sail. Rig them up and see that they sheet in correctly. In any case you should be able to use the bag for either.
As to the sheets, they do run to the back of the seats on my boat and it works well enough. The pressure is more forward than up. I had to reinforce my blocks by putting bolts through both sides of the seat tubing. I did see one boat that put an additional block forward to give their crew additional purchase on the spin in heavy winds. This is probably the reason for your second set of blocks near the crossbar. The sheet is a single line that runs through both (or all 4) blocks and around the forestay/halyard to the bag on the deck.
I installed my windicator on the bottom of the pole just back from the tip and I haven't caught the sail on it yet. I'm not even sure the sail can actually reach it except during a bad takedown, like forgetting to release the tack and dropping the halyard too fast. You can also catch shrimp this way.
Keeping the sail off the forestay: You will probably run your sheets outside (on top of) the spin halyard. This will force the sail around the front of the pole during a gybe. You can gybe the sail "inside" or between the luff of the sail and the forestay, but most don't. Unless the air is really, really light, the chute should remain flying well away from the forestay during a gybe. The skipper controls how much pressure you have on the chute by how fast and clean the boat makes the transition.
Two other things: The asymmetrical spinnaker has a luff and a leach, it will NOT do well if you fly it backwards. Hang the sail and you will see that the tack is longer. I'd recommend that you label the sail so you can leave it in the bag when you tie on sheets, tack and head. This leads to the second point, the length of the sail needs to reasonably match the distance between the pole and the spinnaker tang. If the sail is longer than that distance, it will not work well. The sail should be a bit short of the tang allowing you to tension the sail against the prebend in the pole. If the sail is much shorter than that distance it will still work, but I expect performance would suffer.
Everything said, it really is much easier done then it looks and sounds. If you want, you can contact me at NWDIVERTODDatGMAILdotCOM and I'll give you my phone number. We can walk through it with you looking at your boat.