1) Your KPROBE results are not a definitive and scientific way to determine if a disc will be playable or not. In fact KPROBE results may VARY with different models of drive. You can get a good reading on a 1633 and a bad one on a 811S, it all depends on how GOOD your reader is. Of course if it gives bad readings on different writers and those known to be good readers then you have a problem. ALSO aside from running quality tests, run a speed/transfer test and look for a smooth curve. Any spikes and changes in speed like for example a sudden drop in speed then back up, etc...... means usually it is media related. Also when scanning look for CRC errors and read failures, usually those discs will fail verify. You may get a not so good KPROBE reading on your DVD writer, however, your standalone DVD player may be able to better read the disc, hence a better reader, so it should not have any problem. You will need a lot of trial and error because there is no definitive answer.....if the DVDs work on YOUR player and that it passes the transfer test with a smooth curve, then you should be fine - stick to using known good media for your given writer as well. You want the disc to be readable after a month
2) The DVD Player upon inserting the disc will read the booktype of the media to determine if it is a pressed DVD, recordable DVD or if it is NOT a DVD and a CD, in order to execute the appropriate command. Some older DVD players do not recognise -R or +R or one of them. As to -R, it is currently the most compatible, over 80% (maybe more) will play this format. Of course the +R has more advantages, other than being able to bitset, it is more tolerant to error. By changing booktype to DVD-ROM you are FOOLING your DVD player into THINKING it is reading a pressed DVD - however, keep in mind the DVD specs were optimized for pressed DVD - whilst it is true that your -R DVD player or a non -R/+R player will very likely be able to read a +R bitset to DVD-ROM, there is always the possibility (although rare) that it will not or have trouble reading the disc. It will also depend on the MEDIA, which plays a big role. Certain players will not read a DVD+R bitset to DVD-ROM, but that is rare..... By using DVD-ROM booktype you will get much greater compatibility across players, but not 100% of the time. However you lose nothing in doing so. There are also other advantages of using booktype as you can also fool your DVD-ROM and access your disc faster in some cases.
Even if you use DVD-ROM as a booktype, it is the combination of media quality, how the disc is authored, and many other factors that determine overall compatibility. Luckily most drives now are very leniant and not 100% in spec so you might get away with it - Some players however are very picky about media and other technical details.