I am not sure if eating half a gram to see if they are poisonous is that worthwhile. Personally I would never eat mushrooms unless I trusted the source 1000%, but since I have always picked my own I guess I am in a position where it is easy to apply that standard. I was under the impression that fatally poisonous mushrooms took several days to a week to take effect, by which time noticing the toxic effects is probably too late to do anything, I am going by memory here and that could be wrong, and obviously would vary depending on the poisonous mushroom. Half a gram of a relatively shit batch of shrooms might not even be enough to notice much effects and the inactivity could cause you to believe you have ingested poison when you have not. My best advice is to only take mushrooms if the source is completely trustworthy and if in doubt, throw them out, as opposed to taking a small dose to find out if they are poison by inducing toxic effects on yourself.
I would say that 3 grams of dried subs is a hefty dose, but not ridiculously over the top or anything. Most of my mates who trip maybe a handful of times a year would take 1.5-2 grams of subs for a moderate to fairly intense trip (no ego loss or anything though), obviously they are relatively experienced but they certainly don't have the tolerance of a weekly tripper or anything.
It really depends what you are looking to get out of them, if you want a super intense experience and have proven experience keeping your shit together under hefty doses of psychedelics then 3 grams might be the dose for you. Equally, if you are only looking for a moderately intense experience, and arrived at the figure of 3 grams by assuming subs were the same potency as cubes then yes you should definitely take less.
Psilocybin is certainly a less forgiving drug than LSD in my experience, it can take you to some pretty dark places and I would rate mushrooms as more intense than LSD on basically every level. The only thing LSD has on mushies is the longer duration.
Given the things you are going through in your personal life at the moment, I think any dose of mushrooms is likely to give you some level of trouble/discomfort at some point during the trip. There are certainly those who are of the opinion that some of the most difficult ones are the ones that you also take the most away from, personally I like to err on the side of caution and do my best to avoid any potential negativity though. Only you can decide what level of effects you are after and so it makes it hard to advise exactly how much to take, for a lot of experienced trippers I am sure 3 grams would be the ideal dose but given how unforgiving mushrooms are it could equally be a big mistake for a lot of people. The fact you will have benzo's on hand does give you a bit more freedom to dose highly without as much worry, but obviously they wont completely cure a bad trip.