I can see both sides of this, tbh. The Prem is trying to fit in a full season in a truncated period of time already, so it's hard to see where they can pull in any time to bring the final forward apart. That said, there should be some give in terms of releasing non-final players for the Lions. My gut feel is that PRL are loath to do that as it would open up the demand for greater access to non-English players outside the International Windows from the other nations, jeopardising the money they receive from the RFU for access to English players outside the Windows (The RFU will say "You released players free to the Lions, why won't you do that for us?").
As for the game against Japan, I think it's a good thing in that Japan missed out on England touring this summer. I was personally looking at going on the official supporters tour as it would've been something different from the usual locations. It will also give the Lions squad some gametime before reaching SA.