Yes Newcombe had few time to train in late 1974
however he was not so out of form as he was in early 1972 or early 1973. In late 1974 Newcombe had won the Japan Open over Rosewall, the Australian Indoor Open, the Gunze tournament in Osaka. And above all he had played two weeks before the '75 AO, the Masters at Kooyong on the same site as the AO where he beat young Borg and Parun (but was beaten by Vilas and Nastase). So the situation was different from 72 and 73. Newcombe in late 1974 was near the top while in early 72 and early 73 he was an ordinary player beaten by every journeyman (except at the AO 73 but there he didn't meet any true great player). In late 74 Newk was still a top5 player (and in my mind the #2 for the whole year behind Connors and ahead of Borg and Rosewall). So his win over Connors at the 1975 AO was not so surprising. Until that AO 75 Newcombe had always beaten Connors : US 73, WTT 74 (July) and AO 75 : Connors's first win occurred only in the April 1975 Las Vegas Challenge Match.
About Kodes, of course no one can claim he would have won for sure however he was clearly at a disadvantage and Newk himself recognized that Jan had played unbelievably for two sets and besides Newcombe wasn't probably as strong in September 1973 as he was in early 1974
so Kodes could have a great chance in this final had he not been cheated on this very important point of the second set.
Apparently every witness except the linesman saw the ball clearly out. Yes it is possible that there were other bad calls in favour of Kodes however none hypothetical bad call was as important as that one given it enabled Smith to even the score (1 set all instead of 0 set to 2) which is hugely different.
I liked the Nastase example who effectively thought almost exclusively of bad calls during his matches.