just quickly and not precicely as you requested:
"Well priced" dial type thermometers are often "out" so I dont really use them for this purpose. I use a reasonable quality digital thermometer. I push a foam takeaway cup up into the group ring area, poke a hole just above the bottom and just big enough to get the thermometer probe into the cup, and push it in.
Activate the group and read the temperature at whatever part of the pour you wish to measure but remember you only need to pour out very little say no more than 60 ml. (Theoretically 30ml for the brew and another 30 mll to account for water that would have remained in the puk)Aim to read somewhere around 90 to 92 degrees C. but the spec is around 88 to 93 degrees depending on whose "bible" you read. Loss in temp is quick so dont just stick the thermometer in a cup on the drip tray, and dont use a cup that will absorb heat from the water.
We generally expect a reading in the mid to low seventies for the actual brewed espresso temperature in the cup, the temp loss is great, this would be considered normal and is why everything should be prewarmed. Temperature loss is your enemy.
The group solenoid is not an adjustable item, it is no more than an electric tap that simply opens or shuts off the flow of water when electrically energised.
You are probably referring to pump pressure and whilst many will tell you to adjust or modify so you can adjust the pump pressure, I have never seen a silvia that I couldnt brew a decent espresso out of as set by the factory. Yes it may be improved, but what I am getting at is that it is unlikely that any standard factury setting will result in your machine not working properly except if it has actually "broken down" in some way.
Flow and pressure are not necessarily related and adjusting pressure up or down will not I think increase or reduce flow. The flow from your machine is governed by the designed set up of the pump, and the backpressure to that resulting from the fineness of grind supplied, the size of the dose supplied, and the pressure of the tamp supplied (by you, in the filter).
Looking specifically at the last sentence of question 3, I would be concerned that you may be barking up the wrong tree in chasing machine problems that may not be present. It is very possible this may be due to operator technque. Do you live near any coffesnobs or anyone for that matter, that could quickly check your machine (not with instruments) and simply advise whether it is working properly and doing what it should?
Hope this helps.