Yes (but obviously not before you actually get the instruction to do so).
If you are not under speed control, that can help, yes. However, when you are under speed control, it is very likely that you will kept at a higher speed (as an example: here in Germany, controllers should not instruct below 200 knots before 12 NM final unless necessary for separation and it is very usual to keep people at at least 220 knots until they are established on the approach procedure), so this shouldn’t be the only tool for you to rely on.
Also, controllers should know your turn radius for a given speed (rule of thumb: GS/100=turn radius in NM), so unless they mess up, there should be enough space for you to complete the turn if you execute it promptly. Conversely, reducing your speed too much in this situation might result in your turn radius decreasing so much that you it becomes the cause for you ending up with less than the minimum straight and level segment.
You should not descend before establishing on the lateral part of the approach procedure, correct. But unless ATC specifically tells you to maintain an altitude until you intercept the vertical profile, once you are established on the lateral profile, you can descend with the procedure (charts will tell you minimum altitudes for each leg).
You can increase your rate of descent to catch up with the vertical profile, that’s perfectly permissible as long as you as the PIC deem it to be safe. In fact, ATC may even ask you if you are able to intercept the glide slope from above if they vectored you in too tightly, if you overshot and ATC wants to know if they can turn you back immediately even though you would end up above the normal profile, or if you didn’t descend as fast as ATC expected.
Yes, if you feel that it will be too tight, let ATC know so they (together with you) can figure out a solution. However, if you need more than the 1 NM straight and level (or 2 NM if it’s a non-precision approach), you’ll have to let ATC know well in advance and not when you’re already on an intercept heading.
Perfectly reasonable! Just keep in mind that many ATC instructions (particularly in a terminal environment or on the ground at an airport) can be really time-critical and in general, on busy frequencies (of which we have quite a lot on VATSIM as controllers usually cover much more airspace alone than any controller ever would IRL), ATC needs prompt readbacks because they usually have a couple other instructions already lined up that they need to give to other aircraft, so promptly reading back and complying with instructions can be very important.
I personally have a hardware MCP which allows me to turn a physical heading knob while reading back an instruction, but that can obviously be tricky with the default HOTAS setup many simmers have where you need to turn the heading knob with the mouse. Something you can do here would be to have the mouse already on the heading knob in anticipation of the turn, start turning in the instructed direction while ATC is giving the instruction, then once ATC has finished talking, immediately read back, and only after that finish turning the heading knob (unless you can get it set up completely while ATC is still giving the instruction, of course) - that way you already start the turn and continue turning while reading back.