I had the backstay, boomvang, and outhaul at slight tension and the jib track at middle position, which I thought were appropriate for the conditions.
Can anyone please advise?
What probably/possibly happened during the gusts was that you became 'cross controlled' --- the forestay became slackened due to the elasticity of the wire and became curved WELL off to the lee side (because of the slack slight backstay tension) ... this causes severe degradation in the ability to 'point' (even during a close reach). Your boat was now probably skidding to leeward (which shows up as excess heel). You attempted to correct by releasing the mainsheet which caused a 'power-up' in the mid or top panels (due to increased curvature and deepening of the amount draft which 'relocated' more forward than normal). all 'cross controlled'.
Most 'cruising sails' are designed for usage in 12-15 its of wind strength, and the shape of the sails, especially jibs/genoas 'depend' on the tension of the forestay .... the sailmaker will ASSUME that you forestay is nominally at about 15% tension. At higher wind strengths the forestay stretches and the luff is now operating well to the lee side of the boat (apparent lee helm), when you correct by mainsail or rudder cross control .... the boat now begins to skid to leeward.
I think in this case and from the descriptions given, the MAIN problem was the forestay wasnt tight enough for the conditions at the time, and the center of the jib luff was wellllllll off to leeward (luff to leeward, draft great increased and the jibs leech section was 'hooked up to weather'. Since you apparently 'reefed' the jib (sail area reduction) did you ALSO move the jib fairlead car FORWARD so that the sail didn't become overly twisted (an aver twisted sail can suddenly power up, especially in 'gusts').
In especially gusty conditions and well above that 12-15kts design point what was 'cut' in your jib, you really need to increase backstay tension to match the 'worst' windstrengths at that time to avoid 'jib shape and trim problems'.
Heres a hint on how much backstay tension needed ... simply by watching how much 'curve' that forestay takes at max. wind strength (gusts). Once you get that amount of forestay curve under control, then all the CLR/CE stuff then can be assayed/applied.
How to detect a leeward skid: watch the stern wake, look for the zone of turbulence coming off the rudder at the stern .... if its not coming fairly straight off the boats stern (4-5° maximum 'off angle') the boat is either skidding to leeward (felt as 'weather helm' but isnt) or your 'weather helm' is actual
(a 'sail shape and trim, etc. problem).
Heres how to assay the correct amount of backstay tension to have a sailboat have a fairly 'neutral' helm even in 'blammo' conditions. Just dont much above 30% backstay tension as thats where the stainless wire/rigging components starts to 'yield' and accumulates a LOT of fatigue. Once you go above 30% wire tension, you MUST reduce sail area/reef, etc.
A sailboat with a too loose forestay/backstay simply will not 'point', even on a close or even a beam reach.
Setting correct backstay tension -http://www.ftp.tognews.com/GoogleFiles/Matching%20Luff%20Hollow.pdf