curling

If you work several rows of any reversible stitch (including garter - nice border for all sorts of patterns) the END will not curl up. That will have no effect whatsoever on the sides though. Stockinette knit into a long rectangle will curl in at the sides no matter what you do if it is long enough. Borders help on a short piece or on an afghan or something that will lie flat on a surface like a bed or sofa, but gravity will work against a long scarf. I have a lovely piece of stockinette-based lace knit by a friend which has fairly wide borders. It becomes a tube immediately when it's held up at one end or thrown around the neck.

Curling is simply inherent in the structure of stockinette stitches and that's one of the reasons it works so well for sweaters, hats, mittens, socks, i.e., anything that circles a part of the body.

garter scarf

Actually you are better off with a garter-stitch scarf anyway. It will lie perfectly flat.

It is more compressed vertically though, so a piece of the same length and width will take more yarn and will take more time. The stitch to row ratio for stockinette is usually something along the 5:7 lines whereas it's just about 1:2 in garter. But that makes garter items nice and warm as well.

When you need a break from the garter project, practice a lot of swatches with scrap yarn (a couple of small skeins in different colors from a dollar store will go a long, long way). Work with stockinette until you are really comfortable and then work with ribbing (switching between knit and purl stitches on the same row). Garter, stockinette, reverse stockinette (just the backside of stockinette) and ribbing form the basis of the bulk of knit items. Once you can do them to your satisfaction, you can learn to do a couple of simple increases and decreases and then you can make just about any basic articles of clothing.

Could you possibly have done

Could you possibly have done two knit or purl rows in sequence? That would switch your stitch-pattern orientation. If you are seeing purl bumps and smooth vees (aka "reverse stockinette" and "stockinette" stitch patterns) in the same row it could only mean that somehow you have actually combined knitting and purling in that row.

This is one of those situations that might be easily identified if your work could be seen. But it is very difficult to imagine if you are sure that you are doing everything according to instructions. Can you post a picture and provide a link?

loose purls stitches

It is not uncommon for purl stitches to be knit looser than knit stitches. But how big are your holes? And has your stitch count remained constant? And how could you tell that the holes were the result of your purling and not your knitting?

Large holes and a stitch count that grows mean unwanted yarn-overs. If that's not a problem try shoring up your purls a bit.

Now for the real fly in the ointment - if you haven't noticed it yet, a stockinette scarf will curl in toward the purl stitches at the sides and up toward the knit side at the ends. A very short scarf will have less of a tendency and can be controlled by using border stitches of a non-curling nature (garter, seed). A long scarf will curl no matter what you do because gravity will defeat borders. There are lots of reversible patterns available which take a little concentration, but which are merely combinations of knit and purl stitches. They are best for scarves.