Would the military conceal it? They could probably achieve this if they tried.
Honestly? I doubt that they would, thinking that it would give them the popular support to take complete control of the government over the last shreds of civil resistance.
A worse Pacific War might lead to the US demanding a Soviet DoW on Japan to continue receiving L-L. and to host XX Bomber Command from Soviet bases
No. Stalin was not going to get involved in a two front war. Especially not when the Japanese posed no real threat to him. Not when Germany was literally at the gates of Moscow. At best, they turn a blind eye to bomber crews who had to divert to the USSR after bombing Japan.
Plus, the US wouldn't have realized that the campaign is worse than OTL. They were already having to root out Japanese garrisons almost to the last man. And they didn't demand it off the Soviets then. They wouldn't now.
I agree that Stalin likely wouldn't declare war, but also that the US wouldn't ask. However, a no-holds-barred Japan might still impact Lend-Lease, if the Japanese actually started shooting at the L-L vessels...though this could probably be averted by transferring the ships to Soviet flag.
Down the road, assuming that the Japanese refuse to surrender--which I don't think unlikely, given that in a certain sense the Americans have killed the Japanese nation, even if unintentionally--the US high command and President are going to have a hell of a choice to make after the bombings fail to provoke a surrender. Namely, do they commence Downfall themselves, or let the Soviets do it in exchange for keeping some or all of Japan in their sphere, assuming the Red Army, with L-L'd Higgins boats and US naval assistance, could do so. The latter is more politically feasible in the short term, but it will in the long term and in the halls of power make Truman look super weak before the Soviets. On the flip side, a Soviet invasion of Japan could turn into a resource sink...