1. 1 kV was a common rating for small capacitors in tube days.
2. 6800 uF, 25 V. Yes, that is a V.
3. No units = microfarads C13 is 0.22 uF.
4. Because picofarads are 1,000,000 times smaller than microfarads, and writing 0.000000047 uF on a part is hard.
4a. Sloppy reference designation practices. There actually is a written set of standards for using the abbreviations of scientific quantities. For example, volts, ohms, henries, and farads are not capitalized when spelled out, but the abbreviations, V, <omega>, H, and F, are capitalized because those quantities are based on proper names. The abbreviation for siemens is S, but for seconds it is s.
But - people take liberties in schematics to make things more legible. It is common to see all capital letters in all abbreviations.
ak