I too have been worried about the screen falling off if the adhesive strips turn out to be low-quality. So I took the precaution of internally using a cable tie coupled with two adhesive-backed cable tie-downs (like what you would use to fix Ethernet to a skirting board) so that if the screen should become detached, it can only fall forward maybe two inches before being restrained by the cable tie (http://macs2u.co.uk/imac-screen-restraint.jpeg). Hope that idea is useful to folk, could save serious embarrassment and expenditure.
Looks like it can be upgraded: see http://dosdude1.com/highsierra/ This excellent software allows later versions of OSX on ‘unsupported’ Macs. He comments: “The 2007 iMac 7,1 is compatible if the CPU is upgraded to a Penryn-based Core 2 Duo, such as a T9300” Guy