The shifter has no bonus feats so you're going to spend all of yours on grappling probably. At a minimum you need improved unarmed strike or dirty fighting, improved grapple and greater grapple. You can also get into one of the styles which aids grappling; Snapping Turtle or Kraken are styles you could have started by this point. If human/if you take a dip in another class (like master of many styles monk or unarmed fighter) you could have two or even all three feats in your chosen style.
You can become immune to grapple combat maneuvers by taking your swarm form which I think means enemies can't try to reverse control of the grapple. Probably not an intended effect, it occurs because the writers were too conservative in handing out the swarm abilities and restrictions.
There are some magic items which help, notably anaconda's coils is a belt which gives the constrict ability.
The shifter has no bonus feats so you're going to spend all of yours on grappling probably. At a minimum you need improved unarmed strike or dirty fighting, improved grapple and greater grapple. You can also get into one of the styles which aids grappling; Snapping Turtle or Kraken are styles you could have started by this point. If human/if you take a dip in another class (like master of many styles monk or unarmed fighter) you could have two or even all three feats in your chosen style.
You can become immune to grapple combat maneuvers by taking your swarm form which I think means enemies can't try to reverse control of the grapple. Probably not an intended effect, it occurs because the writers were too conservative in handing out the swarm abilities and restrictions.
There are some magic items which help, notably anaconda's coils is a belt which gives the constrict ability.
Honestly, the ability to not have your grapples broken? Insane. There are lots of ways to boost grapple CMB, but CMD is a much harder one to raise up. Without looking closely into a build, the idea is intriguing, although as you pointed out, pretty feat intensive for a Shifter.
It is probably worth it just for the 5 levels to snag that ability. 8 minutes a day should be plenty.
As for the giant spider idea, swarm shifter has no polymorph abilities. They would need to pick up shapeshifting through some other ability.
Yeah, cavalier edicts are a mechanic. Follow them or risk losing your order abilities. Note that tying up your enemy requires the use of rope or something; your GM may be willing to refluff what you're doing I guess.
You only have a couple of levels to use here if you want that level 5 ability, and since that's the main advantage the swarm shifter gets with grappling I recommend doing so. So no, large is as big as you get.
There are no significantly cheaper means of getting constrict. You don't necessarily need to do so though, grappling is good at disabling enemies even without making you extraordinary at damage.
For 1-2 levels in dips I'd mainly look at the feats you get, although master of many styles monk has the potential advantage of getting two styles which you can have active at once, and/or getting a different grappling style which you otherwise wouldn't qualify yet like grabbing style.