I do use spray satin poly over Tru-Oil on all my hunting bows. I like the protection I get with Tru-Oil and have used it for many years. I used to buff the Tru-Oil finish with steel wool for a satin finish but after use the shine came back. I began spraying a light coat of satin poly and been using it since.
There is no reason to add spar varnish over Tru-Oil. I don't believe it will add any extra protection. A spar varnish finish on it's own, should give you good protection for your bow as will a Tru-Oil finish.
No matter which finish you use on your hickory bow it will still take on moisture and release moisture as the humidity changes. This usually doesn't happen over night but over a period of damp or dry weather. By keeping your bow in your air conditioned(or heated) house(or you have a hot box) you can control the hygroscopic rate and maintain a lower M/C for your hickory bow. Moisture effects all wood bows. Some woods are effected more than others, unfortunately hickory seems more effected than other bow woods. No matter, I still think hickory is one of the better bow woods and I live in an area with high summer humidity.
