mischiefdebs has good suggestions. I would clean it up but without losing the natural, rugged look. Change the base and platforms. Don't waste it on cats 'though, put potted plants on the platforms. Definitely don't put it in the garden unless you want it to rot away in no time. Don't despair, it has great potential. ;-)

None of them – too busy, too contrived and really the 'art' feels dismissed and unimportant (and I suspect that that is probably the case – if it were important to you because of beauty or sentiment or genuine interest it wouldn't be relegated to an over-busy wall just to complete some grid or slightly random pattern). I have a lot of paintings, drawings, etchings and photographs around my house but each one is there because it has meaning and adds to my life and aesthetic pleasure. When they have to share a wall I spend time ensuring that each is still able to be seen, appreciated and respected in its own right.

Ann - have you checked photos of photo arrangements on Houzz? There are thousands of them and many ideas for creative arrangements too with different size and shaped frames and variations in mat sizes, plus you will get a better idea of spacing too. I also note the theme of the photos seems to be important for a good result - family photos, holiday photos, bird or animal photos, etc. and also colour, too much colour could result in a daub that confuses the eye. Another arrangement using this sized frame I really like -

Hello Elpi, I really love how you have chosen to display the mirror and artwork on the chosen walls. In regards to the buffet, I think you will find the space isn't blending nicely because it may be a little over crowded. If possible, I would see how it looks mounting the mirror to the wall, and changing to decorations with noticeably different heights. You could still use some of the items that you already have, but possibly opting for a few more that are shorter and rounder, as opposed to tall and skinny. A good place to start might be adding items that are similar colours to that of the artwork. I also, I think this would also help the dining room merge with the living room, which seems a lot softer and doesn't feature as many straight lines. Look for small, rounded coloured decorations, similar to the attached images as inspiration. Good Luck!