1) Currently, there’ no option to add %% around the text, you need to manually type %% around the search text.
2) The “Filter” icon in the Result window refreshes the existing filter and provides an option to create a new filter.
In your case, once a filter is created, you can use the existing filter to add/modify it by selecting the corresponding cell in the table data and right-click on it and select the “Custom Filter” option from the option-list (“Filter -> Custom Filter”) to retain/use the filter. Please refer to the screenshot, here
3) We have added the feature request to our tracker, here, but the priority is yet to be assigned.