Would we still have removed Mossadegh and Arbenz?
It could go either way. It depends if one regards the change in US policy towards Iran and Guatemala between 1952 and 1954 as evolutionary or revolutionary.
Truman had supported a regime change attempt in Albania, anti-communist partisans behind the iron curtain, fiddling with the Italian election, and possibly a coup in Syria in the late 1940s (it is unclear if the US instigated and plotted much, or if the coup plotters gave US people in country, and they decided not to report them and then prepared to work with the post-coup regime of Husni Zaim.). So it's not like dirty tricks were beyond the pale for the most recent Democratic administration. At the same time, at the conclusion of the Truman administration, it saw Mossadegh as reliably anti-Soviet as anti-British, and thought that the British trying to dictate terms to him via sanctions was a destabilizing policy.
Would we have taken a tough line against the Europeans on Suez?
Probably. Not sure why Stevenson would not. And it could be butterflied before it starts, by other events. For example, if the US never comes around to the British view that Mossadegh must go.
Stevenson might try to rein in Batista's consolidation of dictatorial powers in Cuba. Although he may not bother, or any pushing might be too meager to gain any results.