First of all, you misunderstood..I'm not a painter actually...lol

1. I'll recommend Jotun's Jotaplast or Pacific NewLite..Pacific Paint is a newly incorporated company and to be frank with you, it's(Pacific Paint) emulsion paint is of one of the best quality in the market..
And yeah, Jotun indeed have the Multicolor Tinting Machine and from this machine, you can almost get every possible color in your mind. But for the cheap emulsion paint, only certain light colors can be mixed with the machine..And this also depends on which shop you buy from..Not all of the shops know how to mix cheap emulsion paint using the machine..
In fact, quite a lot of other companies also have this type of machine..
For example,
KCC Paint
Sissons Paint
ICI Dulux
Nippon

2. Yes, if the paint is concentrated, then you should add a little of water. The 1 part to 10 parts, I also confused edi. don't understand la..But it is advisable not to add too much of water and normally 5% is ideal..But some paint have different amount to dilute..For example, Jotun's Solvent-based Sealer(Cito Primer 09) can be diluted with about 50% of solvent. Sometimes, more than 50% and sometimes less than 50%..

3. 5L can cover approximately 10 sq. m..Some can cover less than that and some can cover more than that..But average, 10 sq.m..Gloss paint can cover more than that around 13-17 sq.m depending on the painter or the person who painted it..

4. I'm not that good in masking tape but as far as i know, any masking tape also won't peel off the paint...You can buy one masking tape and you'll know the adhesive isn't very strong. As far as I know, Sancora's masking tape also not bad..