You have a requirement to page break a table on a particular grouping…you would like a custom heading for the table but are unable to merge the header cells sufficiently to give you the precise table heading formatting you require. Create a list control, assign to the same dataset as your table, add a textbox for what will be the table header, configure the grouping for the list that will trigger the page break (don’t set page break options for the list control just set the grouping), then insert your preconfigured tablix (dataset assigned, fields, formatting etc etc) into the list control.
Now format the table header textbox and configure its value as the relevant field from the dataset (which will be the same field you configured for the list grouping), then set your page break options on the tablix you just dragged inside the list (e.g. page break after).
Run the report and you should have a table and table header that repeats on a new page every time the group changes in the list.