Nice set up you have there, should go some way before you feel the perpetual need to upgrade again
1) I don't recommend pulling single shots as you rightfully mentioned that the basket design is different and hence extraction may be difficult to pull off. Would rather just go straight to the double shot baskets, it'll be better use of your time since you already invested so much in the machines. If you do still want to pull single shots, I did remember seeing a video of someone first dosing the narrower portion of the basket first and tamping it lightly (with a smaller tamper), then filling it with more grinds on the wider rim of the basket afterwards and tamping with a regular tamper. You would need a different smaller tamping tool for this to work though.
2) How you would measure is start off with a brewing ratio, that is how many grams of coffee (your input) to how many grams of liquid espresso (your output). Typically I like to start at 1:2 and adjust based on taste from there. If you are not doing a pre-infusion, you would start your timer the same time as when you start pulling the shot and typically you would want somewhere between 20-30 seconds from start to finish.
3) This question requires a deeper explanation and I recommend you to watch James Hoffman's "Understanding Espresso" series videos on them. You can find it here
https://www.youtube.com/watch?v=aTFsBqhpLes. A very basic idea is that the more under extracted your espresso is when it will likely taste sour, you would want to go finer to increase the extraction time and vice versa if it is over extracted where it likely will taste bitter.