Finalising Itinerary before booking accommodation questions!

Hi there again! Am about to see our travel agent to secure accommodation. I have also been talking to other people on the Japan Rail Pass website, who all had varying opinions!! I am also loving the Hyperdia Rail timetable search, and am finding it easy to work out how to get from place to place right down to the very second, which is awesome! A suggestion made by someone was to visit Nikko from Tokyo. We were going to "stop in there" on the way up to Sendai. This broke our journey but I can see that perhaps not the best way to use the rail pass? So I have two questions about Nikko, is it better with the train travel to go up from Tokyo and back down again, and is it advisable to spend one night there? Thank you, for your opinion on that! Someone else also said to head straight to the end of our northern journey first, which is Tazawako. This would mean we would leave Nagano and go straight there, I can see how this would also mean a smooth train journey without much changing. We would then spend a night in there, and then come back out and down to Sendai, stay there, and then our next stop would be Tokyo. So just a question regarding Tazawako, is this the pick of that area to stay in? What would you recommend as a northern stop in May in that region? My other question that no-one answered on the other site was regarding the train travel from Takayama up to Toyama and across and down to Nagano. Is this a scenic trip, or is it mostly in tunnels due to the mountains? Thank you. Apart from this, our trip really consists of sticking to the major bullet train routes to the main centres and is very simple. Would you agree that the following order of travel uses the train system more efficiently? Kyoto-Takayama-Nagano-Tazawako-Sendai-Tokyo-Nikko-Tokyo-Hiroshima-Osaka. We have decided to catch the train straight from Kensai Airport to Kyoto on arrival and spend our first 3 days in that area (settling into your beautiful country), instead of starting off our journey travelling further to Takayama and doing a couple of one nighters. It also works out that we can start our rail pass a bit further into our holiday leaving some of it for our time in Osaka at the end. I so appreciate your help and am ready to book our accommodation, but before that I must have this itinerary nailed!!