How to make a Turkish Delight Syrup

I've worked with agar a lot, and made a few things from the HRC. Of all of the things you've identified as possible problems, I think the biggest likely ones are the quantity of agar, and the agar powder itself. Everything else about the cooling issue is normal. Agar powders do vary. You want to find one that is pure, unadulterated agar, not one that is setup to be a pre-prepared dessert as those are weaker.

If just for a laugh you want to try to save this batch, heat it back up until the agar fully melts, shear in a second amount equal to what you did last night, making sure it fully dissolves, then let it set again. I'm pretty sure that will work.

You can also pick up a sub-gram scale on Amazon for around $20. Thanks, drug dealers! They are very helpful when making small batches of "molecular" recipes.

Grainy means your sugar formed crystals during the heating process. Lemon and other acids retard the formation of crystals, which is why one came out better than the other.

I'd suggest adding some cream of tartar to the other one (which should do the same as the lemon, but without changing the flavor).

In general, to lower crystallization, you should make sure the pots are absolutely clean, and then take great care not to stir or jostle the mix during the heating process. You can also add a little corn syrup, if you roll that way (adding acid actually inverts some of the sucrose into glucose and fructose, which is why it helps to add acid, but you can also just add glucose, in the form of corn syrup. I wouldn't use honey, because of it's water content.)

Hope that helps.