Okay, lots of points to answer.
In the first minute of the last mission, the Albertine goes stealthy for a second or two, and if you read the previous Command briefing, you should be aware that the Sensors are responsible for the Albertine's stealth. Therefore I expect you to realize that you should learn where the Sensors are. You have time to do it as you approach the fighters for the first dogfight - actually, this is the PURPOSE of the initial dialog/approach stage. In an earlier version there was even a "Which subsystem was it again? The sensors, right?" kind of message, which was meant to imply this further.
I can confirm the bug. The mission indeed doesn't end when you fail the mission. This wasn't so the last time I replayed the mission and failed it. Looks like I accidentally managed to cut that end-mission SEXP. This oversight alone justifies a patch, but I'll wait for some more issues people discover so I could make a comprehensive patch. Meanwhile, added to OP.
The difficulty of the last mission greatly depend on the orders you give. I am aware it could be easy if you fly well and give sensible orders.
It always help if you keep your wingmen alive, and the balancing of most missions requires you to do it. Mission 7 especially where you have a time limit.
The destroy Sensors directive is there. Unfortunately though new directives have a tendency not to appear. I even increased the number of displayable directives from 5 to (I think) 7.
The stealth fighters don't disarm the Washington as quickly as two wings of bombers are supposed to. However, if you don't pay attention, they may kill one or two beams, and the outcome of the duel may depend on it especially if you weren't too good in the first stage.
You are not supposed to always need Delta's help. They help a lot though.
The MX-70 - like any heat-seekers - is well usable in front combat. From other directions, it's not too good, unless you know more about heat-seekers' behavior than I do (possible).