Since no one has really touched on this part of the question:
'Will [...] dry hopping post fermentation help a no head/thin beer?'
No, and in fact dry hopping would probably only serve to make things worse.
As Mr_road notes, it's the isomerized alpha-acids that contribute to foam stability. Dry hopping adds basically no isomerized alpha acids since isomerization requires high heat or pH or a catalyst to take place and the dry-hopping (and hop storage) environment should provide none of these.
What dry-hopping contributes is primarily essential oils, which will dissolve in the ethanol in the beer. As with any kind of lipid, these oils should do nothing but decrease your foam stability, though it's not likely to be a huge effect.
Aside from the essential oil fraction, dry-hopping will also provide water-soluble tannins, or polyphenols, which, in high concentrations, may contribute astringency to your beer. It might help the flavor, depending on your taste, but the first thing that comes to my mind when thinking of a remedy for a thin beer is definitely not adding astringency.
While I'm at it, here are a few other things you might consider as being potential causes of a lacking head:
- Assuming you meant your starting gravity was 1.055 and it finished at 1.005 (not 1.55 and 1.05) that's ~6.6% ABV. Alcohol has a fairly strong foam-negative effect above just a few % ABV.
- 10% oats in your recipe may have contributed considerable lipid to the wort, and its survival into the finished beer would depend on your particular methods, especially for wort separation (running off) and trub separation (knocking out). Doing either over-rapidly or carelessly could lead to elevated levels in the final wort and subsequent beer. Grapefruit peel and coriander seeds are also fairly rich in lipids/oils.
- Your chosen yeast strain and fermentation parameters (pitching rate, oxygenation, temperature &c) may have had an effect: yeast-derived fusel alcohols, esters and fatty acids can have a foam-negative effect. Since you used an ester-y (Belgian) strain, especially if you fermented warm, this may have contributed to the issue. Also, high levels of simple sugar at the beginning of fermentation (from the honey) may have effected the yeast's fermentation performance further.
- Since you describe it as thin, I'd have to assume it's probably not as viscous as other, more full-bodied beers. Viscosity is one of the more important determinants of foam-stability (as viscous liquid drains more slowly from the foam), and is probably roughly related to finishing gravity. This is the reason adding maltodextrin does increase head retention.
None of these are necessarily the cause of this lackluster head, but some combination of them together may have helped create the issue.
This turned into a much longer answer than I had intended, but I hope you find some of it useful anyway.