Why Britain tolerated American expansion in 1898

Would anti-German feeling push the United States toward France if Germany organized European resistance?

Not automatically. France and Germany both had awkward Mexican precedents, while Britain was America's closest European partner and preferred an orderly transfer of imperial control to contagious independent revolutions.

French and German involvement in Mexico complicated the assumption that France would naturally support the United States against Germany. Both powers carried political baggage from the intervention that installed Maximilian, so continental alignments were not straightforward. Britain was the United States' closest European partner at the time and repeatedly discouraged outside intervention. London could accept American control replacing Spanish rule more readily than an unaided colonial revolution that might inspire unrest elsewhere in the British Empire.