I'm not very familiar with NextGEN but hopefully this will help.
(1) Smaller Images. I bulk converted my images with Irfanview setting the output size to 50% then zipped them (one gallery per zip file) and uploaded them. NextGEN can resize images Gallery-->Options-->Images. I've never used it so can't comment on how well it works.
(2)To add or edit the image text/description I used Kompozer wysiwyg editor to set up my description and copy/pasted the html source over to NextGEN (Gallery-->Manage Gallery then select a gallery).
(3) "clicking on a thumbnail & going to the image with text, then clicking takes you to the jpg." I'm not sure how I did this - eek! Maybe it's the default? I do note however, that after looking at 2 or 3 other NextGEN users galleries only mine worked this way. On their galleries clicking on a thumbnail takes you straight to the jpg. I guess this is controlled in the gallery options - I can make screen shots of my settings if you need them.
(4) You mentioned editing taking place in a php page. I wasn't sure if you meant the descriptions which is covered in point 2 above or editing the gallery-caption.php file. For the later I just went to Plugins-->Editor selected the NextGEN Gallery plugin then selected nextgen-gallery/view/gallery-caption.php and edited it there after copying/pasting the existing fle into notepad as a backup first.