Can you miscarry and not even know?

Yes. Approximately 10 % of all pregnancies can be lost even before one considers obtaining a pregnancy test. This so called biochemical pregnancy is common to all women. The pregnancy loss is too early to see on ultrasound.
Yes. I agree with others. Research shows that about 40% of all human pregnancies are lost before the time of the menses. Perhaps another 10-15% are lost in the week or so after menses are late but before we can see the pregnancy on us (biochemical pregnany). We do not have consensus among docs regarding what to do if these happen over and over. Best wishes.
Not necessarily. I would see a doctor if this is the case. A variety of things can cause swollen lymph nodes, and it is important to figure out if the the cause is infectious, non-infectious, cancerous, benign, or something else. If a node itself (excluding the skin around it) is 2.25cm, that is almost one inch and surely big enough to worry about.
Yes. After you have implantation...A blood test caled HCG will be picjked up in blood...If this goes away, then you are miscarrying a biochemical pregnancy. It isn't until you qre 7-8 weeks that you can see a baby in the uterus...If you lose this that is a clinical pregnancy so answer is yes...
Yes. If you get pregnant but the pregnancy does not develop in the first few days after implantation, your pregnancy hormone level can go up and down before you would ever realize it, and only delay your period by a few days. The levels of pregnancy hormone can sometimes even be too low for pregnancy tests to detect.