chansey over blissey

1) and 2) are certainly true. The difference in motivation decay is huge.

As for 3), Chansey is worse than Blissey but better than anything else. The total time to knock out a Blissey is about 3 minutes including loading screens, for Chansey it's 25-30s less. Chansey deals significantly less damage to the attacker as well.

It's about your preference and playstyle. If you're usually the first to take down a gym, Blisseys are preferable. For slotting into gyms that are already your color, a good Chansey is certainly valuable.

I maxed that exact Chansey myself not that long ago. It's nearly ideal, since there's no difference between 14 and 15 stamina IVs for Chansey at level 40, and even if it does a little less damage, arguably that contributes negatively to its survivability anyway (although the difference in practice is probably negligible). I live in a fairly active area and Chansey is a lot rarer to see in gyms than Blissey, while still being a potent defender, so it's convenient to always have something to drop in a gym that will actually contribute to defense (your point 1). I have a similar set of other defenders as you as well, which I will usually place first if I can, but having a maxed Chansey is quite annoying for attackers.

You already have two Blisseys. Chansey is the clear choice.

I always want to have 10+ High level Blissey to drop in a gym. Chanceys are second best so having 10 of those is great to have. So I would build a bunch of Blisseys first then build a chansey team. I only drop Blissey, Chancey, and Snorlax in gyms. Unless I’m quickly passing by I drop a Slaking because he is always at the top of the list.