I've never heard of letting the acrylic 'rest' before counting sts. It is a good idea to wash and/or block your swatch the way you will the finished item. Acrylic generally isn't blocked so you can wash and dry it. And make your swatch larger than 4x4; the edge sts/rows curl and aren't the same size as the center stitches so it's hard to measure accurately.
A lifeline is generally used in more complex patterns such as lace. You would thread thinner yarn or thread through the sts at a point you know there's no mistakes below it. If your gauge changes, lifelines don't help with that.
If by cable needle you mean a circular, some of the interchangeable sets have a small hole you can fasten thread or dental floss in and leave it in the sts as you knit.
Most patterns realize you'll use the edge stitch for seaming so you don't need to add one. Keeping it in stockinette probably works the best, or just whatever the pattern stitch is. You can knit both fronts at the same time, just use different yarn ends for each.
Don't be scared, the worst that will happen is you have to rip out and start again.