Taxis in Cuzco are all ONE price--3 soles???

If you have excessive luggage, they might charge you five soles!

What the heck is a 'peso'?

A measurement of weight? :-)

Sometimes taxis are 2 Sols, at night they charge more. At the airport, don’t accept taxi rides from those inside the airport compound who want to charge you 10 or 20 sols, walk up 50 or so metres to the gate onto the main road and wait a couple of minutes and flag down any passing taxi, should be about 5 or 6 sols. Keep in mind US$1.00 is about 3 Peruvian Sols.

Re the train, you need at least to book 24 hours in advanced. With 24 hours notice, we got tickets but they were in different carriages. We got the cheapest train, the Backpacker, costs about US$80 return, leaves Cuzco at 6.15am and arrives Aguas Calientes (the nearest town to Machu Picchu) at about 10.15am. Comfort level is of western standard, Mom will be fine.

Don’t book accommodation in Aguas Calientes, or you’ll pay quadruple. Plenty of the whole range of accommodation, just wander around the small town and get what you want, leave Mom and backpacks in the nearest café while you do that.

Get up early the next morning and take a cut lunch (restaurant at MP is expensive and not good food) and catch one of the many buses for the 20 minute ride to MP.

Taxis are 2 soles anywhere within the city. After 10 PM it's 3 soles.

Well... I think that they are three soles on the further out suburbs, agua buena or san sebastian. For example, magisterio is the cut off line. See a gringo, and the taxista would charge 3 soles... I'd argue two and get it.
My advice would be - negotiate before you get in because they will still try to charge you more than two or three. And unless you are a foreigner earning peruvian pay rates then you can probably afford to pay a bit more anyway.