In the OT Satan can only do what God allows him to do (see Job).
This is true. There are some OT prophets that were so close to God that He bent the rules a bit, just for them.
We see this with David and Job. David committed two mortal sins, and the people were not allowed to stone him for this. God had a separate covenant with him. This only because David was a man after God's heart at a level that warranted this sort of special treatment.
Same thing with Job. God knew that Job would be loyal to Him despite all the terrible things that the devil would do to him. God gave the devil power to do anything bar killing him Job 1:12.
Now there is a lot to unpack on this. God does use the devil. Thats a fact. You are correct that the devil is very limited in his interactions with us. Let's call 'us' the common person

. Do you want to be as close to God as Job? if so, maybe God will put you through the same? I don't and can't. God knows this. That is why I will report to Job in heaven one day. I have no problem with that.
It seems Satans main cause is to tempt us to sin and thus exercise the freewill God has given us.
Yes correct. I believe that Adam and Eve would probably have eventually eaten the forbidden fruit themselves. The devil simply sped up an inevitable fall. God certainly does want a balance of good and evil and the devil helps with that.
He cannot attack us or do whatever he wants more than God allows.
Correct.
Does it make any sense to anyone that an Omniscient God is going to create and empower any divine being to the extent that He puts his own immortality in jeopardy? Would you?
Well God is such a good, kind, humble and loving Being that I believe it is not that He won't make someone else powerful for 'selfish and prideful' reasons. But rather that it is simply not possible for a creation to ever be Him.
The devil has a lot of power and even in his state has still not been robbed of it. Same with every fallen angel. That can still walk around with their heads lifted high. Albeit they pulled the middle finger to God and were removed from heaven.