You lost some accuracy with your calculation method. Better to use Artan or Arsin as the actual length of the link does not change as you displace it This gives around 0.41 degrees.
Adding caster can help straight line stability but also helps turn in by increasing camber on the front outside wheel as you turn.
Increased front toe in to the maximum end of the spec and rear toe in to the middle to maximum end of spec helps also and off course bump steer adjustment.
I have always found my Plus 2 more sensitive in a straight line to varying road surfaces and throttle application than my Elan. i have put this down to more movement of the longer suspension arm on the bushes. One day I will try poly bushes in the Plus 2 to see if that helps.
The tyre world has moved a long way since ACBC first setup the Elan and Plus 2 and I am sure he would now be doing something different. If Lotus had any commercial and historical sense they would develop and sell a non adjustable suspension upgrade / rebuild package developed to suit modern tyres for the Elan and Plus 2. But I guess not enough money in it from their perception.
But if they factor in what this would be worth for their current car sales advertising
i.e
"Handling is in our DNA and we support the handling of our cars for 50 plus years as our 50 year old cars are better than most modern cars in terms of handling"
I think they would be sensible to do it. The did an upgrade/ rebuild package for the later Esprits a few years ago but unfortunately did not do one for my S1 so I am stuck doing it myself when I have time
cheers
Rohan