Trouble seating tire bead
I have a (borrowed) pair of "vintage" 26" Araya CV-7 rims and Schwalbe Marathon 1.5" tires. The rims are marked 1.5" width, and (measured with calipers) are about 18.5mm in inner rim-to-rim width. (Per Sheldon Brown's tire sizing guide, these tires should be slightly on the wide side for these rims, if I read the chart correctly.
My problem is that I am having great difficulty getting the bead on the tire to sit properly all the way out on the rim; it seems every time I mount the tire there's some part that's stuck a bit too far inside the rim. (As a new user, I'm unfortunately not allowed to post a photo...)
I'm aware that this is a somewhat common problem, and I have tried the usual tricks one reads about online—carefully massaging around the tire, overinflating, using windex or Schwalbe's lubricant to try to get it to seat properly. I've improved the situation, but not fully fixed it.
I took it to the bike shop, and the fellow there just told me to spend a lot of time massaging the tire (and gave me a free bottle of Schwalbe's lube). He himself spent a few minutes with the air compressor trying to get it to sit right and was unable to. (I could drop it with him until it's fixed, but in the future, what will I do if I have to patch a tube?)
As an aside, when I rotate the tire on the rim, the stuck-ness does not stay at the same point on the rim, so I do not believe it's something wrong with the rim. It may stay at the same point in the tire, but this occurs with both tires, so I do not believe it's a defect. (I speculate that it may be that the rim tape is to blame, as mentioned on Sheldon Brown's page on wheelbuilding...)
I have two questions.
1. Any tips on how to properly seat the tire?
2. More critically: I've gotten this to the point where it's close enough that the ride is fine. (At more extreme off-ness, the ride is lumpy.) Is riding on this a safety risk? My sense is no; if the bead were outside the tire there'd be a risk of a blow-out, but with it stuck farther in, I think it should be...fine? Right?
(Yes, I am at the point in my life where I'm asking random people on the Internet for safety advice. Why not?)