Armando,
1)     Common practise is using total float. It identifies critical and near critical depends on how you set it up. Best practise recommend to avoid constrain which distort the critical path. Total float also recognise multiple path based on the logic while longest path only identify the longest path from start to finish of the project which generally total float equal to zero.
2)     Same answer on number 1 as explain.

Armando,
The longest path is calculated by identifying the activities that have an early finish equal to the latest calculated early finish for the project and tracing all driving relationships for those activities back to the project start date. It is the longest path in the network. Generally Critical Path is the longest path having total float of 0 but not always the case. If an activity is constrained Critical Path is not the longest path or if having an option total float define larger than 0. Another if having a multiple critical path then critical path is not the longest path.

“Here are some text taken from AACE International Recommended Practices:”
Longest Path
“Longest Path” is defined by Primavera Systems, Inc, as the string of activities that “have an early finish equal to the latest calculated early finish for the project“. Primavera CPM software identifies all driving relationships for these activities and traces them back to the project start date. In general, the Primavera Longest Path only considers “forward pass calculations” and returning controlling relationships and does not concern itself with “float.” This process includes activities that might otherwise be left out by the lowest float method definition of critical path.
Primavera defines a driving relationship as: “a relationship between two activities in which the completion of the predecessor activity determines the early dates for the successor activity.” Software such as Primavera Project Planner (P3) uses an asterisk (“*”) in the predecessor or successor window to denote the fact that a particular relationship was a driving relationship. Longest Path has been implemented in all Primavera CPM software including SureTrak, P3, Contractor, and Primavera Enterprise (currently also called P6.) This RP assumes that the algorithms used in each of these implementations are identical and give identical results.
Just as with the CPM, the determination of the Longest Path is a process. Using CPM forward pass calculations, it identifies the last activity in the schedule and then works backward identifying all longest path activities using their driving relationships. This list or chain of noted activities comprises the Longest Path.

Primavera’s Longest Path identification is only the starting point for this analysis. Consideration must be allowed for:
• Limited inclusion of activities. Only the activities identified as being on the Longest Path are noted.
There is no provision for analyzing how close other activities are from being included in this list.
• Sensitivity to constraints. Constraints (such as start-no-earlier-than) that affect the forward pass calculation also affect the Longest Path. Constraints of this nature can invalidate the Longest Path by causing it to occur only in the future and not proceed from the actual data date.
• Sensitivity to CPM calculation modes. Primavera’s Longest Path is determined based on the schedule calculation method selected. If the schedule has been calculated based on progress override, the Longest Path only identifies activities that have not been affected by out of sequence progress.

Longest Path Value
The longest path value method identifies the longest path from the status date of the project to completion by a manual method or by using a third-party software product. This technique uses a number instead of just an on/off indicator to identify membership on the longest path. The principle used is similar to the Primavera Longest Path method but differs from this approach in the following ways:
• No logical relationships are ignored, even those to completed activities. In effect, this means that longest path value always uses retained logic CPM calculation mode. Progress override CPM calculation setting will cause the longest path backward trace process to ignore predecessor relationships from started activities.
• The “forward pass” is made without consideration of constraints. The consideration of constraints in the longest path may cause the longest path to begin much later than the status date. This violates the basic concept of a continuous path from the status date to project completion.
• Lag durations are always computed using the calendar of the predecessor activity.
• The backward longest path analysis begins with the designated substantial completion activity. If a substantial completion activity has not been designated, then the longest path should begin with the activity with the latest early finish. The longest path must proceed through the substantial completion activity in order to be valid. Any activity that logically occurs after the substantial completion milestone should be not be considered as being on the longest path.

Note: Longest Path was introduced in P3 for Windows Version 2.0b and is now implemented in SureTrak as well as the Primavera Enterprise software.

Best Regards,
Rodel