The defenses were, but were the troops still there, and were they still the elite forces that had existed? Were the aircraft still there? Was there still fuel for the aircraft there, how many of them could still fly (assuming there was fuel)?
The Seventh Division had already lost one Regiment of its best troops at Tenaru River and Edison's Ridge thanks to Colonel Ichiki's ill-advised attacks while in command of the 28th Regiment. Additional troops had been bled off for different missions (including the invasion and garrisoning of Attu), replaced by late war conscripts of much lower quality.
Moreover the 42nd was concentrated on the northwestern tip of the Island near Wakkanai in defensive positions lacking transportation some 150 miles from the Sapporo Bay region. The 7th Division was concentrated to the southeast, generally in the area of Obihiro, over 100 miles from the Sapporo area. Even today that is a difficult trek, especially if most of the troops need to move by foot. The defensive position shown near Sapporo are 32 miles apart, incapable of mutual support.
Certainly the force that are still in place (and considering the way the Japanese were bleeding troops, the difference between paper strength and actual ration strength is likely to be significant) will fight to the best of their abilities and extract a considerable toll, maybe even one that would lead an American or British planning staff to reconsider. We, however, are not talking about the WAllies, we are talking about the Red Army and Stalin, both of which were effectively numb to casualties at this point. As I have noted previously, it is probable that the Soviets would see far more value in areas like the Korean Peninsula and Manchuria which are there for the taking without causing friction with the country that was still providing considerable amounts of materials to the USSR. That the Soviets would not move because of concerns about unrecoverable losses from an understrength division (closer to a brigade in all probability, with two regiments = attachments) and a reserve division with deficient equipment.
Regarding the Soviet ability to make a landing it is worth considering that the Soviets managed, with no WAllied support to land better than a division in the Kuril Islands, against a forces of similar strength and composition to those on Hokkaido (Kurile defenders were, on paper, close to three divisions, the 89th and 91st ID, an independent tank regiment, an air defense regiment + attachments). The initial Red Army landing was outnumbers about 5-1, but much as was the case on Hokkaido, the IJA forces were short on materials and with sub-standard training. This allowed the Soviet forces to establish a beachhead that could be reinforced, while the Japanese defenders were entirely on their own.
At the beginning of the war the 7th Division consisted of three regiments, the 26th, 27th, and 28th (the 25th Regiment was sent to form the core of the Sakhalin Mixed Brigade in 1940, which later became the 88th Division in 1945). Of those three the 28th was more or less destroyed in the battles of Guadalcanal and Attu and had to be reconstituted in the homeland afterward. The other two regiments, to my knowledge, stayed in Hokkaido for the rest of the war and therefore the 7th should have retained most of its equipment and motor vehicles; the only further comment I can make is that when it was demobilized it had an unusually low roster of combat personnel for a division of this type, only 10,800. As for the 42nd, it consisted of 14,599 troops and the 101st IMB had 5,484. The subtotal for Army ground personnel on Hokkaido (excluding the IJN, which also had a significant presence there) was 101,029, with the balance being made up of a mixture of static fortress units, army artillery, construction, and supplies personnel.
As for the aircraft, they too were what was on-hand at the time of surrender and were turned over to the Americans. Giangreco states that since at least 1944 the Japanese had been taking pains to establish a strategic fuel reserve in each of the main islands (mostly on Honshu and Kyushu) to be tapped only for the final battles and deemed sufficient for carrying out the Ketsu plans, so I can only assume that there was at least enough for one-way missions on Hokkaido.
Maintenance and reliability given the deteriorating resource situation was an increasing problem for the Japanese late-war and many would-be suiciders had to turn back on their Kikusui missions at Okinawa because of engine trouble. Though the distances around the Japanese archipelago weren't nearly as great as those from Kyushu to Okinawa, that problem certainly wasn't going away. I can say, however, that in Hokkaido there were three airbases considered to be of high-quality by the US (Bihoro, Obishiro, and Chitose 1) that possessed permanent installations including dedicated maintenance and repair facilities. The complete map of Hokkaido airfields is as follows:
Whether or not these were sufficient to keep
all 450 aircraft in flying order is probably unlikely, but it still would have been a serious threat to the Soviet fleet.
As for the Fifth Area Army's actual ability to deal with an amphibious invasion, say, at Rumoi, the immediate terrain is very mountainous, making it quite difficult for an attacker to fan out. Furthermore the region is directly linked by railways, that, if allowed to remain intact, would have permitted both the 7th Division in the east and 101st IMB in the south to send reinforcements to the area in short order to contain the opposing lodgement, basically Anzio-style. Probably the only way to ensure that their beachhead could perform a breakout would be to establish others at other points on the island, which would have had to fight their way through the strongpoints near the shore and any local reserve behind them.
I agree that a Hokkaido adventure would have been a distant priority for the Soviets, who were much more interested in carving out a sphere of influence in Northeast Asia and scooping up the natural resources of Manchuria. It's also extremely unlikely the US would have supported such a thing as well, considering the Joint Chiefs (and MacArthur) envisioned Stalin's role as strictly confined to the Asiatic mainland.