Multiple problems in a large score (and several stored versions of it)

(When I tried to fill in the 'tags' box I got an error message).

I am in the middle of trying to write a largeish score of about 100 bars and 14 instruments. I have saves various versions by opening, cutting off the end 50 or so bars and saving under another name. Each time I have done this I have saved the new file and then closed it, and each time I have done this the program has become more difficult to manage. The following difficulties now occur to the extent that I cannot fontimue with the project at all:

1: I added 'cello' twice, and pasted the part 'cellos' into them, then deleted all of 'cellos', went into create/instruments/cellos/remove and the program crashed. Tried several times. No luck. Tried file/parts but found the form could not be filled in.

2: Playback so weak on some parts that I cannot hear it even if I use an external speaker with built-in amp, whilst other parts are strong. Adjusting mixer did between little and nothing.

3: A single note in a chord played between three instruments had one note that showed on the stave as right but played the wrong note. Adjusted up and down until it sounded right but it is written as the wrong note, which may lead to wrong printout. Removing the bar and putting in a new one did not work, and copying and pasting from another place did not either. Neither did copying from another file.

4: It seems that the program tries to refer to the files from which I have copied and pasted - which have often been altered and re-altered a few times - but the more it refers to them the less it is able to allow me to do on the file I an working on.