1) Will Architect 5.0 allow me to burn a 720p to a DL DVD?
Yes. It depends also on your DVD burner’s capabilities.
2) Will a DL DVD maintain 720p or downsize it to 480?
A DL DVD just means two layers of capacity (4.7Gb times two for a total of about 9.4 Gb of capacity). The maintaining of 720p or 480p depends on the authoring program or how you rendered the video. Standard DVD is 720p.
3) If I can in fact burn it to the DL and the DL does keeps it
at 720p, will it be playable on a typical (non blue ray)
HD player?
Yes, but again, it will depend on external factors like authoring software or rendering. A dual-layer DVD with 720p footage will play like a 720p-footage DVD on capable players. There are progressive-scan and “upscale” DVD players, but they will not play BLU-ray or High Definition streams.
I saw a bit of the answers and I think you may be confused with HD (1080×720 and above) and SD/DVD (720×480) dimensions of video. I hope I understood your questions correctly.