The real test will be to put another roll through the suspect camera. Have previous films in this camera been OK? It sounds like a bit of contamination has scratched the film, but this could be the camera, the processing machine, the mounting machine for slides or some dirt trapped in the felt light trap on the film cassette. Another film will narrow it down.

Rechargeable batteries seem to work for a very short period of time - you certainly couldn't change them on an annual basis as I do with Silver Oxide cells (MX, P30T, 110 SLR), Lithium cells (MZ3) or Ultra Alkaline Duracells (645n).
I wouldn't add the extra weight for a AA battery pack with the MZ5N - why not just carry a spare Lithium battery instead?

A 1.5v battery is nominally 1.5v and may well vary a bit, but I wouldn't have thought that was a problem with a winder. I have used Duracell Ultra in my 645n and they have lasted very well - they get replaced after 12 months in any case.
Duracells do have an expiry date on the pack - was this OK? Also, if the winder had not been used for some time were the contacts clean? Also, were the contacts tight on the batteries - if they were not and , for example, the Spanish batteries were physically even slightly longer then they could have contacted where the Duracells did not.

That's about all I can think of, hope it helps!

You need two silver oxide cells, size G13. There is also an alkaline cell that will substitute, but alkaline do not last as long as silver. These are all readily available from photo dealers.

Exactly the same answer as above - ideally, two silver oxide G13 cells. In the MX the batteries are only needed to operate the metering - all other functions are entirely mechanical.

Close up filters have a huge advantage in causing no light loss, but have to be well stopped down for best results. I would expect the Pentax close up filters to be better quality - I don't knoe for sure but I would expect them to be a two glass construction for better results.

However, much better quality wise will be a set of extension tubes. Disadvantage is light loss, but ther is no additional glass to affect the image. Some zoom lenses may not be very happy useed in this way, though.

Apparently fantastic close up results can be obtained by reversing the standard or a wide angle lens (using a reverse adapter) and also by using a short telephoto and attaching a wide angle in reverse on the front of it (special adapter from SRB).

My solution? Buy a second hand SMC Pentax-M 100mm f4 macro lens! Superb lens, useful for landscape work as well.....

I understand that it is very important to use the correct 1.35v cells to avoid inaccurate meter readings and possibly damage to the meter circuit.

As I understand it, you will find three main types of Pentax 67.
The original Pentax 6x7 was improved to include a mirror lock up at which point it became the 6x7 MU (mirror up). It was said that at slow shutter speeds the camera was not suitable for hand holding because the impact of the huge mirror caused vibration and loss of sharpness. However, an article by Ron Spillman in AP suggested that the mirror did not cause a problem. A friend of mine uses the 6x7 professionally and claims to have no problem with mirror vibration.
The 67 and later 67 II are the latest versions and have various refinements whilst remaining basically the same beast.
The main problem with buying 67's second hand is that they are likely to be ex-professional tools and may be very well used.
The 67 is a lovely camera and I only opted myself for the 645n instead because of its sheer bulk and because I would have needed to buy a 6x7 enlarger as well, and the line has to be drawn somewhere.
Results from the 67 are absolutely stunning and if you find a nice one to buy I am sure you will not be disappointed.

Don't let me even try to talk you out of such an excellent buy, or even talk you into it, I think you're doing just fine on your own!

I enjoyed using the 645n very much, but have moved to digital and 35mm entirely. In particular, digital is very strong in macro work, and the results really challenge medium format. I think this is why medium format sales have plummeted and some companies are really struggling.

This does mean, however, that there are some pretty exciting bargains to be found. I think I would be tempted at this point to also look at the Pentax 67.

I've now been using the 645n exclusively for a few weeks and it's certainly a pleasure. One small point, but really very valuable, is the excellence of the viewfinder. It puts the 35mm finders to shame - it's bright, clear and makes manual focusing very easy indeed.
I agree with the comment about zooms - they aren't as good as prime lenses and too heavy and bulky with tiny maximum apertures. Not for me!