[General Questions and Answers] MeteorKing:
Couldn't find solid/non-contradictory advice on this in previous threads, tl:dr version at the bottom.

Putting this in general Q&A because at least the isopropyl alcohol related stuff shouldn't be too specific, but let me know if it would be okay/worthwhile to copy it to the Galaxy S3 subforum (even just the frankensteining part). Thanks. :)

Okay, my mother dropped her Galaxy S3 4G (International Version) into salt water. The screen or phone turned off immediately, but the battery was left in for half an hour before she got it home. I didn't know it was salt water til later so I ran a vacuum over the outside (safe distance for static) and put it in a bag of dessicant packets and rice for 24 hours and then powered it on – I didn't realise until I plugged it into my computer that it had actually turned on, but the screen and speaker weren't working (bottom buttons may work but I can't be sure – on turning the phone on they light up for a split second but don't seem to at any other time, not sure whether or not touching them was working and am not powering the phone up again til I've got the buildup off).

I managed to make a backup of data using kies before opening the phone up and noticing the white buildup that indicates salt water, which she then confirmed. Really my fault for not asking in the first place.

Interestingly, the water indicator in the battery compartment hasn't tripped, but obviously the white buildup means there's no way its getting warranty replacement. There's a huge payment left on the phone, and I would like to try and salvage it – I've got experience working with electronics (albeit not in this state), and if I can get the right information it will probably be cheaper than out of warranty repair. I've tried using a bit of contact cleaner on the connectors and managed to get a lot of the white buildup off the screen connector, but it hasn't helped.

From some reading, I've learnt that 90%+ isopropyl alcohol can help deal with the buildup, so I'll head out to get some high-purity stuff from an electronics store over the weekend, however as most functions seem to be working fine except the screen, I don’t know if soaking the mobo is going to do anything, and most of the screen's PCB is inaccessible (the glass, screen, and screen pcb are glued together on the S3 and part of the PCB is covered), so it's difficult to get to with swabs etc.

Does anyone know for certain whether or not the screen section, which has what I assume is thermal compound, can be soaked in isopropyl alcohol without doing irreparable damage? I have seen entire phones soaked, but the SGS3's screen is attached with glue. If the thermal compound is damaged by this, is it replaceable and if so will regular computer stuff work or do I need something in particular?

Failing this (as it's entirely possible stuff has arced or whatever), I am willing to attempt replacing the screen if I can find a suitable donor, however it's not easy to find 4g versions for parts with intact screens and I don't think we're going to pay out the $170 or thereabouts for a new one any time soon (only way that would happen is if I knew it would work and took the phone for myself, giving her my SGS2).

Will the screen section from an Australian non-4G work, and will it being from a different carrier (unlike the US we don't have carrier specific models) cause problems if I use the 4G mobo? Again, I'd prefer answers from people that know for certain, as even buying phones to Frankenstein the thing back to life isn't cheap.

I assume the speaker is the same thus swappable, right? If not, can it be soaked (I assume not but again seen entire phones soaked).

tl;dr: Intl 4G SGS3 dropped in salt water. Dried, screen and touch aren't working, but phone connects/charges and backup made. White buildup on PCB, so warranty replacement out of the question, want to attempt to salvage or failing that, Frankenstein it together.

1) Can the screen section be soaked in isopropyl alcohol without irreparable damage?

2) Will this damage the thermal paste, if so what do I replace it with? Would scraping it off carefully be possible/work?

3) Will an AU SGS3 non-4G screen assembly from a different carrier (no varied models here) work in it?

4) The speaker is swappable between the two, right?

Would prefer answers from people who know for certain/from experience, if you're not certain please explain your logic (don't want to take unnecessary risks).

Sorry if that sounds pushy, a little bit stressed about this as there are a couple of potential donors on ebay that are ending in the next day or so. Thanks in advance!