I use the same method. The plastic sheets for windows in most model kits are well suited. I have used super glue but it is often tricky to work with (possible discoloration) so lately I've used Noch's Hin- & Weg-Kleber. You can also use white glue. The figures need to be supported while the glue is drying of course. There will be places like in tall grass and rough areas where this method doesn't work.

Yes, all of the above, or tacky sppots. The Woodland Scenics 100 count is $6 US. A half of one is plenty. Then they stay in place and can be moved anytime.

If the figures are to stand on a plastic surface (typically a station or building) then I use Faller super-expert glue as here :









If the figures are not on plastic but standing on ballast or other scenery granules such as here:



or on a soft paving surface such as here :





or soft scenery surface such as here :



then in those situations I use superglue. I find this gives a firm fixture as opposed to the contact adhesive glues such as "Hob-e-tac"