Alyssa,
Stop worrying about your cancer matastisizing! The EORTC risk tables have your chance of progression (including metastatic cancer spread) at 0.2% (1 in 500) over 2 years and 0.8% (1 in 125) over 5 years.
Your cancer grade of 2/4 is probably equivalent to 1/3, but even is it is equivalent to 2/3 it does not appear to change your risk of progression, just recurrance rate (at least according to the risk table data).
Really, the EORTC statistics aren't perfect but when your looking at odds between 1 in 100 and 1 in 500, you probably have the same risk of getting killed in a car accident over 5 years. Not enough risk to worry about and let it ruin your summer.
My understanding is they use the IVP or CT to view the upper tract because occasionally someone had a tumor in the ueretors or Kidney (since its all the same urinary system)... however in low grade tumors like yours it sounds like the standard care is to just do it once at diagnosis and not on follow ups.
Does no stromal invasion mean that the tumor is not in the lamina propria? I believe so, stroma means "connective tissue".
In case of a ta tumor is it biologically possible for it to spread into the blood or lymph system? Sounds like anything is possible, but statistically if you have a low grade single Ta I think your pretty safe. It could be that the only Ta tumors that ever recorded to have metastisized were actually T1 or t2 tumors that were misdiagnosed. Who knows the data isn't that specific or accurate.
Again (disclaimer), I am not a medical professional, so ask these questions to your doctor.