Question for the group. If my british fiance comes over to the US for a legal marriage and we apply immediately after for a spouse visa for me to join her in the UK, will it raise suspicion by applying right away?
Nope, not at all. Many people do it this way.
As long as you can prove the relationship is genuine (by showing you've met, and that you communicate regularly), there will be no suspicion
Looking at the times lines for priority spouse visas it seems like the spouse visa approvals are taking several months while some of the priority fiance visas are coming back in several weeks. what is the best guess wait time for a spouse visa (from the US) being submitted this month/next month?
No, fiance and spousal visas have the exact same processing times - there is no distinction between them.
The only thing that makes any difference to the processing time is whether you pay for priority or not.
Over the summer, during the busy backlog period, both fiance visas and spousal visas were taking about 6 weeks priority and about 12 weeks non-priority.
However, now that they are clearing the backlog, priority visas are taking 1-2 weeks (or less). Not sure about non-priority though.