From a mechanical engineering standpoint (and not necessarily
aircraft!)

Studs are generally used so that normal disassembly doesn't wear on a
non-replaceable soft part (i.e. the cyl head). Studs are stronger for
that reason. My personal preference with mechanical machine design is
to not use lockwashers as they perform quite poorly (such as breaking)
in a severe fatigue environment. The design of a threaded fastener
joint really is best if there is no flex in the joint, and the length
of the fastener is maximized and tightened to a preload sufficient to
keep the assembly locked together - such as in a metal propeller-
crankshaft interface. A marginal joint would be a cylinder base
(where the flange might flex some) or of course an exhaust flange.

Have you tried using high temerature antisieze paste? The stuff I've
used for general purpose is called "Neva-Sieze" as I recall. It is
good at keeping out corrosion even underwater and at high
temperatures. Be careful tightening up a nut with this stuff under it
as it is really slippery. I also use it on spark plugs but again be
sure to torque it only to the low limit.

Stainless has a bad tendency to gall with other metals and
particularily with itself. In addition. Anti-seize should be always
used with stainless for general work - although I don't know anything
about how the turbine stuff is maintained.

Helicoils are effective for getting good strong threads in soft
materials, but once you wreck it, there is no additional fix without
removing an unacceptable amount of parent material. I suspect that
(and $) is the reason they are not used in exhaust attachments.
Fortunately they do use them effectively in sparkplug holes as you
know.