With the second question I meant whether or not max magicka/spell power makes the heal on hit bigger. Meaning if it does, then it is of little use to a stamina build. If it is a standard heal not affected by any stats, then it would be useful for me. I can't test it myself because I don't want to level resto all the way up if it's no use to me.
Force Siphon is Magicka based skill, so initial skill usage will depend on max Magicka and therefor spell power. Though while this ability lasts you gain healing (life stealing) from whatever damage dealing abilities you are doing, which is totally up to you - it might as well be a stamina based attack.