The International Mass Loading Service provides displacements due to four loadings (in decreasing their magnitude): 1) ocean tidal loading atmospheric pressure loading; 2) atmospheric pressure loading; 3) land water storage loading; 4) non-tidal ocean loading. You need to apply all of them.
In general: no. Mass loading has strong changes near the coast line. As a rule of thumb, errors of interpolation exceed 5% within 3 times cell size. In mountainous regions the atmospheric pressure loading may have significant changes within several cells.
If your station is in the list of stations with pre-computed loadings or is located within 2000 m of a station with the pre-computed loading, just download the file with loading displacements. If not, please use the on-demand form.
First, fill the form with downloading pre-computed loading. Set the output type "wget control file". Submit it. Then examine the long URL in your browser. Copy this line. Then find the start and stop dates. Replace the start and stop dates and execute this url as an argument of wget. You need embrace the URL with apostrophes. Command wget will get you the list of wget commands. Execute this list. Then write a simple program (or script) in whichever language your prefer that combines these two steps.
All of them good. Models that failed validation are not put in the International Mass Loading Service. Read the papers with model description, ask your colleagues, toss a coin. The models that I use in routine data analysis: ATM_GEOSFPIT, LWS_GEOSFPIT, TOC_FES2002, NTO_OMCT. I use harmonic variations at diurnal and sub-diurnal bands for all these loadings, except for non-tidal loading. For non-tidal loading I use only the S1 contribution (the variation with period exactly one day: 86400 s).
Depends on the problem you want to solve. These loading variations are real, and their omission in data reduction will cause systematic errors.