(1) if anyone can amplify on this story,
(2) if such 155mm direct fire was an technique which the CA trained for, or if it was ingenuity and innovation in response to the enemy, and
(3) if the direct fire by the CA - ahead of Marines in the beach assault - was part of the operation to secure airfield, for which official documents commend the CA unit."
The battery rates three separate mentions in the official US Army history of the Guadalcanal campaign.
GUADALCANAL THE FIRST OFFENSIVE, which you can download from the CMH site:
The battery along with a battery of the USMC 5th Defense Battalion (also equipped with 155mm guns) landed 2 Nov 1942 at Lunga Point, and "brought in the heaviest American artillery which had been sent to Guadalcanal up to that time, the first suitable for effective counterbattery fire."
Prior to that time, the USMC 3rd Defense Battalion with batteries of 5in (127mm) naval guns for coastal defense were the heaviest US guns on the island. The 1st Marine Division artillery consisted of 75mm pack howitzers and some 105mm howitzers.
The battery was in action less than two hours after it landed against "Pistol Pete", a long range Japanese artillery piece that had been shelling the airfields.
Later on 14 November, two of the battery's guns were used to shell beached Japanese transports at a range of 19,500yds. The 5inch batteries of 3rd Defense Battalion also engaged these transports, at a range of 15,800yds.
So, FWIW, for the rest of your questions.
2 - I suspect that mobile US Army Coast Artillery battalions did receive training which enabled them to support ground operations. Such units did so in WWI, and in WWII such units were later converted to FA battalions.
At various sites you can find US Army WWII Coast Artillery manuals, and all issues of COAST ARTILLERY JOURNAL, from before WWI to 1948 when it became ANTIAIRCRAFT ARTILLERY JOURNAL, which might have articles on this early form of cross-training.
(the pre-1922 issues, most of them, can be found at Google Books as the JOURNAL OF THE UNITED STATES ARTILLERY)
3 - the battery did not support a Marine landing to take an airfield. It helped the Marines defend it by engaging and neutralizing Japanese artillery that was firing on the US airfields.
I suspect that when the US Army launched its offensive against the remaining Japanese forces on the island (in which units of the 2nd Marine Division participated), the battery fired in support of such operations.