I am transferring between Swampy and DM on Monday. I have the option of taking the shuttle bus or go via train. According to the map I can take the train to Mo Chit 2 and then transfer to the Red Line to DM. Due to Bangkok traffic I assume the train is quicker, but is it convenient?

if you have an onward flight ticket from don the shuttle bus is free and easy. takes about one hour from exit #3 at suv.

I don't have an onward ticket so have to pay anyway.

There is no red line to DMK, the map you post will be in effect around 2050AD OR 2600 Thai calendar.
At Mo Chit BTS you can take a bus to DMK, exit 4 and 35B if I remember correctly?

Here in Thailand you see many maps with many pretty coloured lines on them. Some of those pretty coloured lines eventually get built.

From Morchit northbbound to Don Muang, you can use buses A1, 29 and 504. And there are probably others which don't come to my mind just now.