1) I always say this. Quality is a process, you do not "lose quality", you decide your quality parameters.
Sometimes "quality" means speed, on a website, you need to load your images fast, or the viewer will leave.
But on a printed project "speed" is not an issue. File size is not an issue.
2) No. Any aditional compression to reduce file size lose information.
Sorry, this old webpage of mine is not translated in English but it is only to show you a method to analyze this information lose. There are different methods for JPG compression, which gives different results, some of them less file size, but some of them lose more info.
If for some reason your "online provider" limits the size of the files you can upload, first check some other things.
Needed resolution. You probably do not need 300PPI, you probably need only 200PPI.
Color modes, you most likely do not need CMYK, send them in RGB.
You probably need to send the full page in vectors, not in raster format.
As a last resource, simply use PShop to resample a copy of the photos. Do not use any online website, which you do not know the sub settings used to compress your images.
You most likely want to deliver the maximum quality print you can. Do not risk it for smaller file size.
On some more professional environments, you do not use JPG at all.
You start with a RAW image, save it as a 1 bit PSD file, which can be saved as a TIF lossless compression format to be printed.
On a normal everyday life project yes, you can use JPG. You take some photos saved on JPG format directly from the camera, and you can save JPG files using the correct parameters. As @Alex Magill said, some of these compressions are imperceptible to the human eye, (but potentially can cause some issues when printed) but you can still use them as JPG.