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If the texture is a sparyed-on or roll-on application, there are strippers which will loosen and allow that to be scraped-off. If it is drywall mud, it's not coming off so easily and you will spend many hours and create a huge mess sanding that down with an orbital sander... then potentially have to re-skim to get the surface ready to accept a good tile job.
We do a lot of backsplashes in existing homes. When the substrate (the surface to which the tile is to be applied) is not satisfactorily flat and smooth, we find that the quickest, cleanest, simplest solution is to remove the drywall and replace with new, moisture-resistant drywall.
That is going to vary depending on where you live, the particulars of your home construction, as well as particulars of your planned "pop-top". Here in Richmond, VA, the structure and exteriors, plus a window or two for a relatively narrow dormer runs in the $10-12,000 range... before you make interior renovations to the bathroom which presumably you may be seeking.
I am guessing that you may be interested in something wide than a 3-4 foot typical dormer. Well, things become much more structurally complicated in a Cape roof, the wider this pop-top becomes. As the roof loads become asymmetrical, beams and opposing side structural work will likely become involved (ie: you may have to add structure on the OTHER side of the house, and a beam at the peak of the roof... possibly posting-down to the foundation, to balance the load). I have seen those go to $30,000+... again, before you start adding in the bath renovations below. I know; sounds shocking but in order to maintain the structural integrity of your roof, these things may be neccessary.
There are exhaust fans that have a filter and move the air in the bathroom but do not vebt to the outside, this takes cae of the odor. To address the mold you can repaint the bathroom ceiling with a quality paint like Emerald from Sherwin Williams and add a mildew inhibitor to the paint. (it is a liquid that you add to the paint that stops mold from growing) We have used it several times in bathooms and our clients have never called us saying the mold is back. I would also highly recomend leaving the bathroom door open after you shower so that the steam does not stay in the bathoom.
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