There is no ideal plan. Tumblr don't like advertising regardless of what company it's with.
Place any ad on your Tumblr accounts and you will lose them ALL. Either setup your own website and push the traffic from Tumblr to it OR build up decent sized accounts and sell them/shoutouts.
And what to expect If using random user-agents + private/elite proxy per account ? That could help ?
What Cazual said is correct.
Even if you do use proxies (which you should with that many accounts) expect that some of them will be shut down. It's a numbers game of making new accounts faster than Tumblr can delete them and learning to stay under the radar.
It's true that Adsense+Tumblr isn't against the tumblr/google TOS ??? How safe is to put adsense in a tumblr blog still in a sfw one... ?
Just get a Bot that can do it for you, Something like Xtumblr.
I have a lot of followers on Tumblr with about 50 accounts. I'm using it solely for ranking my sites. For viral traffic you need a shitload of followers and accounts. But for ranking, a few notes on your posts and you should be able to rank any site with low competition keywords.