Is it safe to run BOV and BPV?

It is plenty safe... you just might leak some boost every now and then. Make sure you ahve a boost only signal going to the CBV to try it's best to hold it shut. So won't break anything, you just might not get optimal performance. I ran like this for a few weeks prior to gettting my turbo. No need to waste you money on a BOP that you will use for a week or so.

I drew this for seomthing else but use #2

red is boost only, blue is boost/vacuum, green is boost/vacuum, but solenoid controlled. That red line comming off of the turbo is actually comming off of the compressor housing.

ignore the other pics.

No, there really is no way to gingerly let off the throttle, once you are in boost, any time the TP gets closed less than it was, you risk compressor stall. All that pressure in the pipe has to go somewhere (into the IM and cylinders) , and if you suddenly close the TP (Even 1%) that air amount that was going through now can't so that pressure needs to vent if it can't it will attempt to go through your compressor the wrong way which is bad for it.

Find a car with a BOV and ask them to go into boost and see if they can't lift the throttle off slow enough to not vent. I would doubt it seriously. And if there are any cars or traffic, lettsing of the throttle slowly might not be possible. Even at 5-10 in hg it is hard to get off the throttle slowly enought to not vent. Just because the IM is in vacuum doesn't mean the charge pipes are still in boost. Same thing, close the TP, pressure spikes and needs to vent.

Though it is your turbo, and nothing catostophic will happen, it isnt' the greatest for the life of it if you continually do it, but if you did it once it probably wouldn't be THAT big of a deal.

Why do you want to block it? Are you testing to see if it is leaking and that is why you think you have low boost? I assume you have aboost gauge. HAve you tried just pulling the neg batt to see if that resets it?