Prior to getting my current crossbow I had and Excalibur. That had a 175# draw and I could decock it by putting on my cocking cord, taking all the slack out on one side and pulling the trigger letting the string down slowly.
Because my current bow has an anti-dry fire mechanism, can I put a piece of dowel in place of the bolt to lower the anti-dry fire mechanism and decock the same way?

bronko i'm not sure if i would try that. all i do is take a bolt with a field point & shoot it into a block target. if i forget the target i just put one in the ground maybe ten yrds out. they do make a small bolt to discharge into the ground. just shatters heard they run around 10 to 12 $ a aluminum bolt runs 4 bucks & can be reused alot of times

Well I tried it and its no big deal. When you're done hunting for the day all you do is remove the bolt, take off safe (once a bolt has been inserted and removed the anti-dry fire mechanism is disengaged) put your cocking string on as normal but pull all the slack to your off side. Because I'm lefty I have all the slack on the right side. Grab the handle on the right (probably be up around your arm pit) and put tension on it. Pull the trigger and slowly lower the string.
I like to have my cocking cord snug on the string by taking up all the slack plus a tad extra so it will stay there until I'm ready to go out again. Then all I have to do is grab the handles and cock the bow.

I can decock my sniper 370 exactly how you described. It does take a little strength, but no more than it does to cock it. Not sure what crossbow you are using, but if it is possible on yours, I'm sure someone has tried and posted it on youtube.

A lot quicker and less trouble simply to buy a decocking bolt and shoot it into a stump or the ground.

It is Oldtimr but that can get expensive! I had a decocking bolt and it lasted two shots. My friend shot his crossbow into the bank with it, retrieved it and handed it to me. I shot into almost the same place and hit a rock which broke the decocking bolt!.
Like I stated in my post above my cocking cord has all the slack taken out of it while it is on the bow and the bow is "at rest" in such a manner that it remains on the string and won't come off. When I decock the bow using the cord I just leave it on the string and it is all set for the next time out. It is perfectly safe to decock in this manner. Watch the video below and you will see how easy it is.
Bzlytbeer I have the same bow.

can this be done with xbow including the cocking device ???

I don't know. I would bet that you would have to have a separate cocking cord. I've never used a Xbow with a cocking device on it.

I keep a foot long piece of 6" pvc, capped on one end, and stuffed with rags in my truck. It's my discharge target. I've been using it for 4 years. It works great. Bolt goes in about 3-4 in. I can withdraw it with 2 fingers.

Grouch, no only with the ones set up for a rope cocker.
Bronko I have been doing it that way for more than 20 years, just be sure to have enough pressure on the rope so it does not thump your toes with the doll.