Answers, in order:
1. Some explanation first. When you run a program such as MS Office, it is read from the disk drive into the PC's memory, and then run/executed by your processor. The XLSX files that you view/create/modify are also read from disk, and manipulated by the MS Office program.
You don't need to restart MS Office every time you open a different file, so you read that program from disk occasionally, possibly only once a day. The many files you deal with in MS Office are opened and created, modified, saved (written to disk) many times when you are working.
Question, how long does it take to start MS Office, and how long does it take to open and save files in MS Office?
Putting MS Office on a SSD will cause it to start faster than a HDD, but you run it once or twice a day. If you find yourself waiting for the files you open to be read into MS Office all the time, then what would save you the most time. What slows you down when using MS Office? I can't answer that for you, only you can. Whatever files you wait for the most, programs or XLSX documents, are best stored on a SSD. If opening your document files from a HDD or network storage is not slow, then use a SSD for the OS and Office.
You could save the documents you work on in a day on the SSD, and when finished, have them copied to the HDD as a backup task.
SRT is really meant to accelerate an OS on a HDD. SRT must "learn" what files you use the most (OS programs) and puts them on the SSD. A XLSX document that you open once a week will not be put on the SSD, but one that you open every day will. Would that really help you?
2. No, SRT will not work with a drive on a network. The HDD and caching SSD must both be connected to the internal Intel SATA ports on the mother board of the PC (can't help but get techie at times.)
3. TRIM should work with a SSD used in a SRT configuration.