The 1930s isn't the 1920s anymore, the era of the warlords have ended and there -wasn't- more than Chinese government (unless you count the japanese established ones) after 1927 with the success of the northern expedition. There was only one government you could negotiate with if you wanted a peace.
Chiang's control was somewhat shaky, though--besides the Communists, there was a warlord rebellion as late as 1936. This is noteworthy as showing that even warlords could play on anti-Japanese feeling:
"It is noteworthy that the last major warlord revolt against Chiang--that
of the Guangxi Clique of Li Zongren and Bai Chongxi and the Guangdong ruler
Chen Jitang in the summer of 1936--called itself the 'Anti-Japanese
National Salvation Army.' According to Eastland, p. 256, originally Chen
proposed 'rebelling in the name of opposition to the 'illegal
constitution' that had been proclaimed on May 5. The Kwangsi [Guangxi]
leaders laughed off this suggestion, however, recognizing that the draft
constitution was anything but a visceral popular issue. Finally, they hit
on the solution. As Huang Hsu-ch'u [Huang Xuchu] recalls, 'But what
purpose should we proclaim? To win the people's sympathy and still
proclaim righteousness, nothing surpassed 'resistance to Japan.'' The
Southwest would, in other words, join forces with the potentially powerful
national salvation movement." Most likely what really worried the
Southwest warlords was that Chiang, by first suppressing the Fujian
rebelllion
http://en.wikipedia.org/wiki/Fujian_People's_Government and then
driving the Communists out of the provinces just north and west of
Guangdong and Guangxi--and keeping his army in those provinces even after
the Communists had escaped to the North during the Long March--was
tightening his control and threatenimg Guangxi and Guangdong's autonomy.
The cruelest blow came when Guizhou, under pressure from Nanking, diverted
the opium trade from Guangxi (Eastland, p. 253). But however cynical the
'anti-Japanese' justification of the rebellion, the fact that its leaders
did make that their banner proved how popular the slogan of resistance to
Japan was..."
http://groups.google.com/group/soc.history.what-if/msg/17db796260af7299
In other words, yes, Chiang was at least nominally recognized as leader of China by all factions by 1937, but he could not therefore make any peace he and the Japanese wanted. His enemies were waiting to jump on him for anything that looked like a sellout.