// pager:true, ver https://stackoverflow.com/questions/46302506/pager-true-vs-pager-someid-in-jqgrid-how-to-use-them-properly

// same page
    var grid = jQuery("#grid");
    grid.trigger("reloadGrid",[{current:true}]);
    var currentPageVar = grid.getGridParam('page');

//load once

    ise forceClientSorting: true together with loadonce: true. olegs version
    grid.trigger("reloadGrid", {current: true, fromServer: true});
// que con
  sortname: 'OrderDate',
  sortorder: 'asc',


// local sort key:value, Check contesto mismos
    colModel: [
        { name: 'ID', hidden: true },
        {
            name: 'SourceLocaleId', label: 'Source Locale', index: 'SourceLocaleId',  edittype: "select", formatter: 'select', editoptions: { value: LoadData(LocaleURL) },
            editrules: { required: true },
            sorttype: function (value) {
                return sortData(value, 'SourceLocaleId');
            },
        },
        {
            name: 'LocaleId', label: 'Locale', index: 'LocaleId', width: 130, align: "left", editable: true, edittype: "select", formatter: 'select', editoptions: { value: LoadData(LocaleURL) }, editrules: { required: true },
            sorttype: function (value) {
                return sortData(value, 'LocaleId');
            },
        }];

    function sortData(key, columnName) {
        var textvalue, getData = LocaleData ;
        for (var i = 0; i < getData.length; i++) {
            if (getData[i].Value == key) {
                textvalue = getData[i].Text;
            }
        }
        return textvalue;
    }

"formatter": "InterFinalPriceFormatter" and the custom defined functions
    $.fn.fmatter.InterFinalPriceFormatter and $.fn.fmatter.InterFinalPriceFormatter.unformat will be automatically called by jqGrid.

// date formatter:"date", formatoptions:{srcformat:'ISO8601Long', newformat:'d.m.Y'}
      formatter: 'date',
      formatoptions: {
        newformat: 'D d/M/y'
      },

// timezone problems
    I see where is the problem. jqGrid gets the local timezone and convert it according to this.

    I suggest you to use a custom formatter in this case instead of the build in one. So replace the formatter : date with

    formatter : function(val, opt, data ) {
        var offset = (new Date( val )).getTimezoneOffset();
        return  $.jgrid.parseDate.call(this, 'U', (parseFloat(val) + offset*60), 'H:i:s');
    }
    el colmodel:

        formatter: date
        formatoptions:
           srcformat: "U"
           "newformat": "H:i:s"

    json: {
      "page": 1,
      "records": 3,
      "rows": [
        {
          "id": 1,
          "name": "FIRST ROW",
          "duration": 1139
        },
        {
          "id": 2,
          "name": "SECOND ROW",
          "duration": 60
        },
        {
          "id": 3,
          "name": "THIRD ROW",
          "duration": 15336
        }
      ],
      "total": 1
    }

// hide empty col
    beforeProcessing: function (data) {
        var i, foundPrescription = false;
        if (data.rows != null) {
            for (i = 0; i < data.rows.length) {
                if (data.rows[i].prescription) { // if not empty string
                    foundPrescription = true;
                    break;
                }
            }
            $(this).jqGrid(foundPrescription ? "showCol" : "hideCol", "prescription");
        }
    }