Micromagnetic structure evolution in Ni-Mn-Ga ferromagnetic shape memory thin films is investigated by means of temperature dependent magnetic force microscopy (TD-MFM). The center of interest is the magnetic properties of epitaxial Ni-Mn-Ga thin films on MgO substrates across thermally induced phase transitions. Experimental results are discussed within the framework of competing magnetic interactions arising in stressed thin ferromagnetic films. Measurements on 14M martensite specimens are supplemented by three-dimensional micromagnetic simulations. Corresponding calculated MFM micrographs are compared to experimental data. The influence of twin variant dimension and orientation on micromagnetic domain formation and wall structure is depicted from a theoretical point of view. A micromagnetic model system of partial flux closure is proposed and calculated analytically to estimate a stress induced magneto crystalline anisotropy constant in austenite Ni-Mn-Ga.