Mira

That muslin is doing its job. All three things you list are normal, named, fixable adjustments and none of them means you picked the wrong size.

• Drag lines running from bust toward the armpit almost always mean you need more room at the bust than the pattern gives. Choose your size by high bust, not full bust, then add the difference back with a full bust adjustment. Choosing by full bust is why your shoulders and neckline are too big.
• The gaping neckline is the same cause: too large through the upper chest.
• A shoulder seam 2cm back is a forward shoulder adjustment, about ten minutes of pattern work.

Do the high bust size next, add the FBA, and the other two problems shrink to almost nothing before you touch them separately. Also, calico behaves nothing like shirting - expect the real fabric to be friendlier.

The envelope will never tell you. Most commercial patterns are drafted for one cup difference, and that single fact explains most beginner fitting despair.

Hand baste. Everyone skips it and everyone has this problem. Ten minutes on a jacket side seam saves the unpicking every single time.

Guide in front, let it feed at the back. If you need to pull, the pressure is wrong.

By hand for this. Machine basting feeds the layers the same way the real seam does, so it creeps in exactly the same direction.

Change the needle again, this time to a stretch needle rather than a ballpoint. They are not the same thing - a stretch needle has a deeper scarf and a slightly different eye position, and it exists specifically for the problem you are describing.

What is happening is that the jersey flags downwards as the needle enters, so the loop never forms for the hook to catch. Two things stop it: a strip of fusible knit stay tape in the hem allowance, or gentle tension held front and back rather than pushing the fabric. If it still skips after a stretch needle plus stay tape, the timing is out and that is a shop visit.

Most packets do not help. Ballpoint for sweater knits, stretch for anything with elastane or anything that flags.

For six wovens a year, no. Buy a French seam habit and a decent pair of shears instead.

A serger earns its space when you sew knits regularly or make enough that the time saving matters. On a 90cm desk you will be lifting it in and out of a cupboard every session, and a machine you have to lift is a machine you use half as often. For a nicer inside on wovens: French seams on anything light, flat felled on anything sturdy, bias bound on curved edges. All three look better than serged anyway.

Stabilise the seam allowance before the zipper ever touches it. A 1.5cm strip of lightweight fusible interfacing cut on the straight grain down each side of the opening, and the ripple stops. Rayon has almost no recovery, the zipper tape is essentially stable, so the fabric stretches to match it during sewing and then relaxes afterwards.

Good news is you already stitched both sides the same direction, which is the other classic cause.