Some good questions here and I'll do my best to answer them
1) For cleaning models there are many approaches and no single one is right. I've used a few over the years:
a) Blade. I typically use a scalpel blade,
GW use to sell a really nice one; however a regular scalpel blade is perfectly good. Swann and Morton make top grade handles and blades (get the none sterilized since you aren't doing anything medical so no need to pay extra). You can easily get them off ebay or any hobby store will generally have supplies of them.
Scraping off mould lines is pretty easy, just be gentle and be prepared to go over a line several times to remove it softly rather than try to cut it away in one go. A scalpel has the bonus that the point is very thin and can get into tight spots that files can't easily get into.
2) Filing sticks/emeryboards/nailfiles etc.... Basically a sheet of sanding paper affixed to a firm back makes a file. Now the bonus here is that you can make your own with some regular glue and some wood (lollipop and cocktail sticks work great - you can even sand down the lollipop sticks to be a bit flatter to fit into tighter gaps). Making your own is cheaper in the long run and you can grade them how you want - ebay often has sets of sanding papers to pick up. I would say a range from 300 to 900 grit would be good to look into. Any coarser (smaller number) and its going to scratch and bite into the material too much; whilst any finer and it can tickle a bit too lightly, though it will be great for smoothing.
3) Files - these come in two kinds, toothed and diamond. The basic difference is that toothed have ridged teeth set in a pattern which perform the cut. They will typically cut only when moving in one direction and can come in a variety of grades - though many don't mention their grade. I would buy quality ones - dirt cheap ones can be poorly cast - I would expect most hobby shops to sell decent quality ones.
Toothed files will get material stuck between the gaps so you need to pick up a brass brush to clean them with - just brush them with the brass brush in line with the direction of the teeth and they'll sweep out the gunk. Soft materials block them up far quicker.
Diamond files are different and will cut in any direction. This can be really good for models because someitmes you've tight gaps so a diamond file can work its way into them and work even if its not moving in the normal back-forward direction. Diamond files also come in various grades exactly like sand paper (far as I'm aware they use the same or a very similar grade system).
Some small 600 Grit files are good - and if you go to the link below you can get a rare 900 grit which are great for smoothing over an area (far as I can find that link below is the only place that sells the 900 in a small size).
Diamond files are cleaned by taking a regular hard rubber and rubbing the file on the rubber to clean out the gunk - it will tear up lots of the rubber, but you can see the difference and feel the performance difference. That said my 900 I really love but they will clock up FAST.
These days my go-to tools are a 900 grip half round file and a scalpel and the blade does most of the cleaning work.
Glue -
GW plastic glue has a slightly poor reputation, which is a shame because most of their tools are generally good quality (if somewhat to very overpriced). Revell Contacta glue is the glue I use currently - good quality and with a thin nozzle to direct it. Note any of the metal nozzles can block with dried glue after use (even if you put the cap on). Simply remove the metal rod and use an open flame to burn the glue out of it. A match or lighter works great for this - just remember to burn both ends. Then wipe off the soot that will gather on it and you're good to go.
Remember if you're doing conversions which involve attaching materials other than plastic you'll want some superglue too - Locktite Superglue Brush On is a good little pot with a brush to stroke the glue where you need it.
You wont need glue to put the greenstuff on (its sticky enough when worked); but if you were attaching resin or metal parts or a fully cured (hard) greenstuff creation you would need the superglue not plastic glue.
3) Greenstuff is great for working into gaps and joins to clean up a model from a conversion. Plastics normally don't need it for general gaps, but resin or metal can benefit from it. I use bot hthe two part greenstuff (regular greenstuff) for larger gaps; whilst liquid fillers I find work best at filling gaps where they've support on all sides. So filling a bubble hole or such. They are less good at filling strips or larger holes - for those regular is great.
Note when working with greenstuff keep the tools you use with it wet - a quick dab in some water will help the greenstuff not stick to the tool.
Also get some colourshapers (also called clay shapers). They aren't much at most hobby stores for a set of 5 or so and they are fantastic for working with greenstuff. They let you smooth it over like you would with your finger, but the silicon heads don't have fingerprints so you get no marks like you would with your finger. They come in 3 harnesses, black (hardest), grey and white (softest). If you only got one set I'd get a set of black.
I keep meaning to write all this up in one big post one day, but if you want a bit more I've got something I wrote for some resin models (most of the methods are the same); it also has the link for the diamond files that I noted above.
General cleaning and construction (I also talk about pinning in this which is a good skill to learn esp for conversions)
https://warminiatures.wordpress.com/2016/01/27/relthoza-vaxiss-ground-attack-leviathan-assembly-guide/
A conversion but its where I talk about the files and link to them - there's also a neat video
https://warminiatures.wordpress.com/2018/07/05/blood-angels-of-khaine-a-conversion/
Eldar- 4436 pts
