The tree has not been pruned over the past few years and it need some serious training. Here are some pictures of the tree:

The main trunk is high, like 4' to 5' tall. The main branches grow somehow to the same direction (south growing). And the main branches are very long, like 5 to 6' long.

1. Can I prune the tree right now? Or I should wait?
2. How hard can I cut the main branches?
3. Will the hard pruning hurt fruit production next year? Or tree training is more important?

Should I try to keep the existing shape (tree shape) or try to train it now as a bush shape? The tree is already tall and I do not want the tree keep growing higher.

You need a plan for pruning. If you were to read various random posts around the web you will see the term "be brutal" but be smart instead. It may appear brutal, but that has nothing to do with the strategy.

Establish what will be your scaffold branches.. these are the ones you will want to have as your permanent sructural branches. you want them low on the tree and ideally 45 degrees (between horizontal and straight up). If needed you can adjust their angle with a weight on the ground and string, in the spring. Those scaffolds should not be blocking each others sun. prune away stuff like that. Also prune away any growing straight up other then your main center, which you can cut it shorter, it will restart itself. You need to pull down the height quite a bit. Keep reading this forum. Magazines, blogs and youtube are loaded with people who don't know what they are doing, but pretending they do.

You will probably want to prune it to an open center because apricots with leaves near them exposed to full sun get sweeter and bigger and the sun helps prevent the fruit from cracking or getting scab if they are susceptible. Also discourages brown rot.

That is a very healthy tree but you may need two to get it to crop consistently. In our region they are not reliably self-fruitful. Where I was raised, in S. CA they were.

I suggest you order an Orange-red from Adams tomorrow morning. They tend to sell out.

The tree guard is from last year's. It is amazing deer has not bother the tree much. So I guess if you put on anything to the tree, then deer would stay away.

Since the tree branched out high and there are strong branches from the center, I'm not sure if I can still train the tree as an open center system. If so, I'll have to cut down a few of the major branches from the center to open up the tree. Or I can keep the existing structure, cut off some of the branches to balance the tree. I know I'll have to cut the main laterals, 1/3 or 1/2? Too much?

If you can pull down branches to a more horizontal position by staking and tying I suggest you do so. There are ways to make hinges that allow very thick branches to bend that would otherwise break.

Do not cut back your current branches to a stub, if you must cut them back to keep the tree in your bounds, cut back to a smaller branch, but such cuts can delay maturity and fruiting.

The tree can be trained to an open center and it is often best to have a high trunk before first branches to keep fruit above the deer and allow the installation of squirrel and coon baffles

My understanding is that Blenheim is not a productive variety here and not highly recommended for our region by any knowledgeable source I know of. However it will function as a pollinizer for Goldcot, which is a pretty good variety for our area.