There are many building inspectors on the net who offer a professional service. In heritage matters we have had good reports about Richard Forrester, formerly an inspector with the NSW Forestry Commission. You can access his website via this link https://paragonengineering.com.au/team-member/richard-forrester/, phone 7227 7775, mobile 0429 646 112.

We assume the alfresco ceiling won't be exposed to the weather in any way. All species of timber react to gaining or losing moisture by swelling or shrinking. If the timber is in a stable environment, tongued and grooved lining boards are available in a number of Australian species from companies such as Austim. You could also consider V-joint plywood or 'engineered' flooring, depending on the appearance you are looking for. Engineered flooring generally comes pre-coated which could be an advantage when installing a ceiling.

It's unlikely that significant deterioration would have occurred after 3 months' exposure to moisture, but clearly the leaks need to be fixed and the timber allowed to dry out. We assume the floor is made from a grade of plywood with a moisture-resistant adhesive. A business that can test your caravan is Southern Independent Building Inspections. Their usual business is inspecting buildings but they have assured us they can moisture test your caravan. Phone 0400 292 226, or visit their website via this link https://sibi.net.au/.

We don't have any case studies on protector strips, but no evidence of problems either. Indeed, our Technical Design Guide #21, Domestic Timber Deck Design, recommends joist protection, particularly where timbers of moderate durability are used. A copy of TDG #21 can be downloaded free of charge from the WoodSolutions website via this link https://www.woodsolutions.com.au/publications - refer Fig. 6, page 8.

A fully detailed answer to your question is a little beyond the scope of our service. Also we weren't sure about the span of the proposed beam. If it currently spans 4.2m between the two pillars in your photo, presumably the span will be greater than 4.2m if the pillars are removed and the beam runs from wall to wall?  In any event, we can give you some general pointers.

We understand the trusses are in South Melbourne. Timber Training at Creswick runs courses on visual stress grading and should be able to suggest some names from previous courses, more info here: https://timbertrainingcreswick.com.au/course/visual-stress-grade-softwood/.