[Matthew Barnes]

Based on my experience, here is a guestimate. Bear in mind that I'm an amateur and not a professional metal man. There are various experts on here that would be able to give you a detailed breakdown. A pro, supplied with decent replacement panels, would be able to do it faster.
The fit and quality of the replacement parts you order has a direct impact on the time it takes to fit and weld.

BUT without seeing the car and carefully inspecting the rust, a pro will be reluctant (or unable) to give you an accurate quote.

1) remove and replace rear parcel shelf? The corners of the shelf seem to be in decent condition. These are tricky. Drill spot welds and cut = 4 hours, grind and prep for new panel = 1 hour, fettle new panel for replacement = 2 hours, weld in = 1 hour, grind = 1 hour. If the brace between the shelf and the rear window frame is rotten then add a few hours for removing, fabricating and welding.
2) replace or patch rear right seat that had rust? [COLOR="rgb(65, 105, 225)"]If you have to fabricate the bottom of the bowl I would estimate 2 hours to to cut, prep and weld with clean-up. Perhaps 6 hours for fabricating the patch. Maybe less if the beater has a wheel[/COLOR]
3) remove and replace front latch panel and weld battery boxes? [COLOR="rgb(65, 105, 225)"]Difficult to tell how bad the edges and bottom are. These are the tricky bits if they have to fabricated to accept the new slam panel. If you are going to order the entire slam panel then the metal work is not that complicated. Drilling out all the spot welds = 4 hours. Prepping and plug weld drilling the new piece = 3 hours. Fitting and fettling = 2 hours. Welding = 2 hours. Grinding = 2 hours [/COLOR]

My two cents. I hope this helps.