JPGs do not get better when one converts them to TIFs. The latter cannot invent what JPG's data compression possibly has blurred. So, use the files "as is".
You have already got the often used thumb rule "do not go under 150 pixels per inch" That's because printing process generally needs that much original data as minimum to avoid the need to make guesses between the printed lines. Doubling that ie. having 300 pixels per inch would be generous. Scaling the image to bigger size with normal resampling methods such as Bicubic or Lanczos do essentially the same as guessing the missing lines in the printing process, so it doesn't help. Normal photo editing programs haven't finer resizing methods, the result will be blurry.
There's one trick that I have used several times succesfully, when one wants to get screen resolution images printed and comes so late that he has no possiblity to get high resolution images. Resolution can be increased - not by using Photoshop's resize or equivalent, but by applying a special image enlargener, which guesses where are sharp borders and thin lines. Everything else (large areas, gradients) are scaled to bigger size, but sharp borders and hairlines stay thin. For ex. about 1 pixel wide cat's moustaches stay 1 px wide. Normal photo resizing to 200% would make them 2 px wide. Of course no new details are invented, but there's densely enough original data for sharp printing.
I have tried Smilla Enlargener (freeware) and On1 Resizer (commercial). Both work ok. Enlargening to 400% has been usable for my purposes. Test them before buying. Here's one of your linked images enlargened to 400%. That's about 51 inches wide when printed at 300 pixels/inch or 102 inches when printed at 150 pixels/inch. https://www.dropbox.com/s/e4c6mozwvgac22m/SeaInSwizerland.jpg?dl=0
You should ask the wall size image printer to show what quality he can produce and what should be the total pixel dimensions of the photo for that quality. Do not believe numbers that you do not understand.